In one of the older episode the kids have an assignment to write about what they want to be when the get older and to come to class dressed like it. When they are in class all dressed up, there is a banner above the window the reads, "Welcome Back Class Of 2000," but when they really do graduate Mr. Feeny says, "I give to you the class of 1999."
In that episode in question, which is the episode where Topanga proposes to Corey, you say Mr. Feeney says, "I give to you the class of 1999.", and what he actually said was "I give to you the class of 1998" not 1999. I think you need to go back and take a good look at that episode again.
Even worse! This has bothered me as well. They were supposed to graduate in 2000, like I did, so I had always thought they were all my age. Then, when they finally did graduate, it was like time had been fast-forwarded, because it was only 1998. So you just proved this point even more so, because they decided to use the actors' real ages, instead of the characters'.