I hate prepositions! I can never seem to choose the right one! Well, here's what I would say:
I'm sorry. I left them at home again.
I was in class at school. (or "I was in the classroom")
Where you at the baseball game yesterday? (BrE => baseball match)
The train is on track 7 (BrE => The train is at platform 7)
My school is on Peace Street in Washington DC (BrE => in Peace Street)
Warning! I am probably wrong, so wait for a native speaker. That's just what I would do... and the differences between AmE and BrE come in part from my dictionary.