More than 50,000 high school and college students who hoped to work for Uncle Sam this summer may have to go elsewhere for jobs unless the Reagan administration modifies its tough federal hiring freeze with the next couple of weeks.

Reagan's hiring ban for the bureaucracy, backdated to Nov. 5, applies to most summer jobs unless they specifically are exempted from the freeze by the Office of Management and Budget. The only summer hiring permitted so far by the freeze order involves disadvantaged youths in various stay-in-school job programs. Federal agencies here normally hire about 14,000 student summer replacements, about 6,000 of them in programs for the disadvantaged, which have been exempted from the freeze.

OMB's guidelines on the freeze require federal agencies to apply for an exemption before they can send out letters promising people jobs, much less put them on the payroll. The Office of Personnel Management could, OMB says, apply for a blanket exemption for all agency summer hiring programs. Whether it would be approved, in whole or part, remains to be seen. p

The federal job freeze comes at a particularly bad time for young people in this one-industry town who want to do more between June and September than mow lawns, baby-sit or watch a lot of television. This is the time when student summer job-seekers, are supposed to be reading job bulletins, filling out forms and making applications to federal agencies where they would like to work.

Details on the summer job program were spelled out here Jan. 7. Basically, students must get a copy of the Summer Job Announcement No. 414 -- from agency personnel offices or the Federal Job Information Center at the OPM, 1900 E St. NW -- to see what jobs are available and what they must do to be considered for them. They also will need a Standard Form 171 (job application) or Job Application Form OPM, 843A. College students seeking specialized summer work in their field of study need to get Form 1170.

Several personnel officials contacted yesterday said they were unaware that the hiring freeze covers regular summer jobs. When told they would need an exemption from the OMB -- and the request must be made by the head of each agency or department -- the majority said they would do so.

This isn't intended to panic summer job-hunters. It is meant to warn agencies that their summer hiring programs for regular employes are covered by the freeze, unless they win an exemption from the freeze. Meantime, if you are looking for a summer job with Uncle Sam, be advised that the competition this year will be tougher than ever before.