STSC Inc., a Bethesda computer services company, yesterday reported a 31 percent increase in fiscal-year revenues and a 25 percent rise in profits for the same period.

The company reported revenues of $27.6 million and profits of $1.84 million (98 cents a share) for the fiscal year ended May 31, compared with 1980 fiscal revenues of $21.1 million and profits of $1.4 million (86 cents).

For the fourth quarter, STSC reported profits of $515,000 (24 cents) compared with 1980 fourth-quarter profits of $411,000 (24 cents). Revenues rose to $7.1 million from $5.9 million for the quarter.

For the first nine months of Penril's fiscal year, the company reported profits of $1.7 million (88 cents) compared with 1980 profits of $1.3 million ($1). Sales rose to $29.5 million for three quarters from $24.5 million.

American Furniture Co. Inc. of Martinsville, Va., reported second quarter profits of $55,747, compared with $402,042 for the same period last year. Sales dropped 11 percent to $17.1 million.

In other reports, Penril Corp. has reported an 18 percent rise in fiscal third-quarter profits, which rose to $526,000 (27 cents a share) compared with $447,000 (34 cents) for the same quarter last year. Sales rose to $10.5 million from $8.4 million.