Were Mary Beth and Julie in high school for 6 years?

I used to love this show as a kid (as distressing as it would be to tune in next season and see half the cast had been replaced) but in watching the YouTube videos of all the openers to see who lasted for how long, it occurs to me that Mary Beth and Julie lasting for all 6 seasons makes no sense. And it wasn't as if you could pretend the 6 seasons of the show only covered 4 years of real time, because every year they would start off like it's a new season and usually included a mention of how all the departing characters are now at college.

So, did they just kind of sweep Julie and Mary Beth's ages under the rug and always pretend they were still high school aged? Pretty lame, although I guess it didn't bother me as a kid.

In other news, my crushes on Mary Beth and Kristy still remain.