Late 70s, playing Adventur, a text-based adventure game, on a SOL-20 running CP/M on dual 8" floppies. My friend's father used the computer to do income taxes, but from May through December we probably put more hours on it than he did. It was awesome!

At high school, we had a Commodore PET and a slew of TRS-80s and teletype terminals. My calculus teacher had an Apple II in 1980 or 1981.