The cost of student lunches in the Cambridge city schools has increased since the end of the last school year almost 20 cents to a anew high of 70 cents per meal, for the most part due to President Reagan's budget reductions.

"They're trying to make it a welfare program," Marilyn G. Tompkins, food services director, said recently, so that only children who still qualify for free meals will opt to purchase hot lunches.

The effects of the higher cost lunches are the most devastating for children whose families earn too much money to qualify of the free lunches but not enough to afford the higher prices, Tomkins added.

200 Fewer Lunches

Margaret Barr, kitchen supervisor at the Kennedy Elementary School, said recently the number of lunches served has decreased by 200--or one-third of the total--from the 1980-81 school year to the current semester. Of the 400 lunches still being served, only 25 percent are paid lunches, she added.

"Seventy cents is a lot of money for a family to pay for each lunch, especially if there is more than one child in a family," Janice M. Oliver, vice president of the Kennedy School Parent Teacher Association, said recently.

And Gus A. DiSano, legislative chairman for the Massachusetts Good Service Association, recently said that in order for the school lunch program to survive, "we need the paying kids to continue in the program."

Officials blamed Reagan's tighter restrictions on families whose children can receive subsidized lunches, as well as a decrease in the amount of direct federal aid to schools, for the decline in lunches served.