After a forced two-month recess, Poland's university students returned to classes today to find a much subdued atmosphere along with a new set of government rules designed to keep the nation's campuses under tight control.

At Warsaw University, one of 70 Polish universities that had been at least partially paralyzed by student strikes for the month before martial law was imposed in December, students attending special first-day sessions were informed that compulsory courses in Marxist-Leninism and the Russian language have been restored and that absenteeism could result in expulsion.

There was a surface air of resignation at the new regulations among students, many of whom said they have been threatened with Army service as punishment for any violation of the new strictures. But while there were none of the vocal student protests that had marked the heady days of Solidarity, it was clear that deep tensions remained.

As the students returned to their campuses, it was disclosed today that Poland's Communist Party, crippled and in turmoil as a result of the country's prolonged social upheaval, has lost about a half million members--16 percent of its total membership--since last July.

One sign of the sense of drift, even at the party's highest levels, is the fact that there has not been a meeting of the party's Central Committee, the top policy-making body, since martial law was declared.

"The situation within the party is still uneven," said Wlodzimierz Mokrzyszczak, an alternate member of the party's ruling Politburo, in an interview with the Polish press agency published today. He said the exodus of party members, which began last year, has continued since December's military crackdown due to extensive government-conducted purges.

The government today, meanwhile, published an ambitious plan for economic and political recovery that is likely to bind the country much more closely to its East Bloc partners and result in a much restricted role for future trade unions.

The reforms were outlined in a long communique appearing in major newspapers reporting on a meeting of the Council of Ministers, the Polish equivalent of a Cabinet, last Friday.

Today's communique promised reforms it said were designed to boost agricultural production and attain the announced goal of making Poland self-sufficient in food. Other reform programs were announced for taxation, old-age pensions, cost-of-living compensation, housing, transportation and other areas, in an apparent effort to placate Polish consumers, who last week were hit with massive price increases for food products.

The government council also ordered development of a plan for a major shift in the structure of the country's factory production to curtail Poland's reliance on imports and its dependence on the West for business and financial credits.

The communique suggested that to avoid independent trade union movements such as Solidarity, future unions would be subordinated to the state. It stated that unions should be "harmoniously connected with the primary aim--that is, consolidation of the state and the socialist democracy."

By outward appearances, Poland's universities, stripped of political posters and slogans, have been made to look as though last year's protests never happened. In the Warsaw University office where the local branch of the Independent Association of Students--the now outlawed student equivalent of Solidarity--formerly had its headquarters, there now hangs a banner announcing the space is occupied by a branch of the Warsaw Yacht Club.

The entryway, once lined with posters praising Solidarity and exhorting students and faculty to action, is now filled with innocuous notices advertising horseback riding lessons and an opera production.

"It's like being back in high school," said Alfred, a 19-year-old freshman humanities major, his white four-cornered cap--a popular student emblem--the only symbol of even muted defiance evident from the heady days of last year.

Another student, Leszek, a history student in his final year, voiced what appeared to be the general view here that it would be senseless to defy martial law in view of the firm hold the military authorities have demonstrated.

Leszek told of a recent incident before classes officially reopened, in which police intruded on a seminar to check student identification papers. Such inspections, the student said, have contributed to a sense of intimidation on campus.

But many students say they cling to the hope that the deep change in political attitudes and outlook of Poland's youth that took place during the past year would prove too durable for the authorities to break.

"What happened to us last year caused us to grow up faster and mature in ways we wouldn't have otherwise," said Alfred. "This will be a lasting difference."

In many instances, too, bonds were formed between students and faculty members sympathetic to the demands for academic reform that may weigh heavily as Poland's schools struggle to maintain their integrity in the difficult days ahead.

The law school dean at Warsaw University was quoted by one of his students as saying in his welcome-back speech today that senior faculty staff had withstood strong pressure from martial law authorities who had wanted teachers to agree to expel students for any infraction of the new codes.

"Where's your commissar?" one student asked the dean in a friendly mocking of a military order that uniformed officers be present to support--and presumably observe-- senior school administrators. One of the small victories at Warsaw University seems to have been agreement by the authorities to keep a military oversight crew off campus.

Warsaw University was a leading center of last year's academic protests, which were triggered by the undemocratic reelection of an authoritarian rector, Michal Hebda, at the engineering school in Radom. The protest developed into a national action protesting government ill will and incompetence in implementing new legislation for university self-government that had been promised following an earlier wave of student strikes in March.

All that is history now.

The Independent Association of Students, which would have been one year old this month, has been dissolved by the martial-law government. It was attacked last month in the government-controlled press here for what was described as its subversive and illegal activities. A number of the association's former leaders have either been arrested or interned.

After enjoying a brief period of autonomy, university senates and departmental councils have been reduced to their previous status as advisory bodies without decision-making powers.

The Conference of University Rectors, which two days prior to the crackdown declared itself independent of the minister of education and responsible for monitoring academic reforms, has also been dissolved for having, according to an official condemnation, "attempted to usurp some of the prerogatives of the state."

Meanwhile, at Radom Engineering School, Hebda is firmly back in control. In a recent interview with the government newspaper Rzeczpospolita, Hebda disclosed that since Dec. 13, almost 80 percent of the school's departmental authorities have been replaced, presumably for defying his orders.