It`s not necessarily true that there`s no socially redeeming value to his hit TV show ''Head of the Class,'' Howard Hesseman says.

''It`s true in my opinion,'' he adds with a wry smile, ''but not necessarily true.''

Hesseman has been playing the plucky substitute teacher Charlie Moore for three years now, and it`s not what he thought it would be.

''We`re not doing the show that I was led to believe I`d do, and it`s difficult for me to get off that,'' he says.

''I don`t want to air dirty laundry in public, but I do feel that the educational arena is one that offers a variety of story ideas as a means of investigating our lives-what we mean to one another and what`s important.''

Hesseman is a rarity in show business: Not only does he say what he thinks, he says it well.

He would like the show (7:30 p.m., Wednesdays, ABC-Ch. 7) to explore the differences between his middle-aged teacher with a childlike spirit and his prematurely aging charges, a class of gifted students.

''We could do all that with considerable humor,'' he thinks, ''not with jokes, but with genuine humor that comes out of character and situations.''

Hesseman, 49, has starred in three TV series. In addition to ''Head of the Class,'' he played the off-the-wall Johnny Fever in ''WKRP in Cincinnati'' and architect Sam Royer in ''One Day at a Time.''

He has guested on dozens of other shows, including ''Soap,'' ''The Rockford Files'' and ''Saturday Night Live'' and appeared in more than 25 motion pictures.

If anybody knows television, it should be Hesseman. But he never watches. ''We`re in our ninth year of a Republican administration,'' he sighs.

''Life is depressing enough.''

Hesseman says he turns on the television only when he wants to catch a particular actor or director or writer.

And he claims to be one of the few people extant who has never watched

''Star Trek.''

He holds out little hope for a fresh approach to television.

''I don`t think they want one. I don`t see any evidence of a fresh approach, do you?

''Sometimes it feels like it`s getting better . . . and sometimes that`s as illusory as Dan Quayle`s television quotient.''

When Hesseman was a student himself in a farm community in his native Oregon, he wasn`t exactly a troublemaker, he says-''not to anyone but myself.''

Still, you get the impression even then Hesseman was not the acquiescent honors student. ''I did myself any number of disservices,'' he admits.

''Certain teachers certainly must have viewed me as a troublemaker. I didn`t like them and I didn`t like the subjects. I didn`t bother with subjects that didn`t interest me. The subjects that interested me-like history, literature, speech, creative writing-took all my attention. In short, anything other than math or science.''

He was asthmatic as a child and couldn`t participate in sports. ''By the time I was in high school,'' he said, ''drama seemed the only avenue left-in terms of meeting girls,'' he says.

Hesseman remembers one teacher fondly.

''We had a principal in my last two years at high school who was roundly reviled by most of the faculty.

''And one teacher, who remains dear to my heart to this day, just practiced this open, running mockery of our principal to the total delight of most of us in his class.

''He seemed to understand that there was fatuous pomposity in this idiot. ''And he let us know that we weren`t crazy to think this guy was a real mental pygmy who was in charge of everything.''

The teachers he remembers best were those who remained interested in their work ''in spite of people like me. They were the ones who didn`t function on autonomic, but seemed to take into account the divergence of their students.

''They were not afraid to admit to their own biases and were willing to reveal their passions about things.''

Hesseman is like that. Sometimes he worries about what he`s doing with his life.

''Part of me says, `Is that all there is? A television series?` Obviously not. I could go on stage or scratch my way into the movies. But how many movies are made that you want to be a part of? And how many want you? There is a certain kind of `for-rent` sign on my forehead. I`m an actor and I like to work.''