"Deadly Lessons" has a slasher movie plot but, since this is network television, no real slashing. Still, the ABC film, at 9 tonight on Channel 7, efficiently generates a healthy amount of suspense, and it keeps one guessing about the identity of the killer right through a decoy ending that is followed by a moderately arresting twist.

One moral of the film--a Leonard Goldberg Production--is, young girls should never attend a private school that goes by a name like "Starkwater Hall," as this one does; it sounds as if it's named after Charles Starkweather, the famous mass murderer. And, indeed, not long after young Stefanie (Diane Franklin) arrives for a remedial summer session, students begin dropping out, in the non-academic sense of the phrase.

To the attentive viewer, it may seem that, as the bodies mount up, the killer may simply have a personal vendetta against young women with eccentric names; victims include "Tember" (Krista Erickson), "Althea" (Sally Klein) and "Shama" (Vicki Kriegler). It's not giving too much away to say the names have nothing to do with it. But do rich people really encumber their offspring with monickers like those?

Donna Reed, back from The Beyond, plays the stern headmistress at the school, a humorless old dictator who is having a thing with one of the teachers, played by David Ackroyd. Both are obvious suspects, as is the weird old caretaker (Donald Hotton) or even the handsome young stableboy (Bill Paxton). A newly transplanted police detective named Kemper (Larry Wilcox, former CHiP) does a truly inept job of sorting this out and protecting the remaining girlies.

Writer Jennifer A. Miller not only knows how to dangle, ever so deftly, a red herring or two, she has even bothered to create believable and multi-dimensional characters and to shade the film with seemingly authentic details of preppie peer pressure and social stratification among the young. The final confrontation should probably have been more harrowing than it is, and it requires the killer to divest himself (or herself) of great reams of 11th-hour exposition ("I suppose you're wondering why I killed all those pretty girls," he or she might have said), but director William Wiard moves the thing along so briskly that implausibilities can be brushed aside like cobwebs.

One other cast member deserves mention: Nancy Cartwright as Libby, the sad plump frump whose father, it is mentioned, is an insensitive U.S. senator who tucked the girl away in private school as a way of hiding her. Cartwright makes Libby sweetly appealing. Her disappearance is not a welcome development. But then, you don't find that many welcome developments in movies about homicidal maniacs. 'Small and Frye'

Our networks are behaving more quixotically than ever. NBC comes up with a roaring, male-appeal hit called "The A-Team" (it leads off Tuesday nights) and chooses stupidly to follow it with the bleary "Bare Essence," thus chasing away the audience. Tonight, CBS pulls a similarly klutzy stunt by following a moderately intelligent sitcom, "Square Pegs," with a test run of an inept and anachronistic prank, "Small & Frye," at 8:30 on Baltimore's Channel 11.

(The premiere of "Small & Frye" is pre-empted on Channel 9 in Washington because the station sold the hour's time to Billy Graham).

Nick Small (Darren McGavin) and Chip Frye (Jack Blessing) are two sticky gumshoes with a silly gimmick: because of a "laboratory accident," Frye can shrink himself down to a six-inch height whenever he wants. So when he sends his girlfriend (Debbie Zipp), also Small's daughter, a box of roses, Frye himself is in the box. Oh, ho!

The laugh-track finds the team's lame misadventures so amusing you'd swear the chuckles were seeping in from some other show, or some other planet. "Small & Frye" is a creation of the Disney organization, which is trying to expand its television output, but Unca Walt in his weakest moments wouldn't have wanted his name on lazy pap like this.

Incidentally, there is one black actor, one, with a speaking part in tonight's show. He plays a florist's delivery man. From a second-story window, McGavin asks him how much the flowers cost, is told "$19.95," balks at the price, then throws money out the window at the man below. The camera stays on McGavin as we hear the sound of crashing cars. McGavin says, flippantly, "Keep the change." Nick Arnold is the writer who thought this was a cute bit. He and the Disney people deserve each other. Nobody deserves "Small & Frye."