"I was still in school then” was showing past tense. "I was younger then" or "That was then, this is now."

I'd have to say the number one argument for grammar would have to be the misuse of the word "at" at the end of a sentence/question. i.e. "Hey Billy, where are you at?"

Don't you know you're not supposed to end a sentence with a preposition?

(Meanwhile, I watch my grammar as I am writing this and second guessing my spelling...)