I came across the phrase
in a comic I'm trying to translate.
I understand the statement means something like "I should be at school!". It looks like 俺は学校に is setting up the speaker as the topic, and that the speaker should be doing something at school.
But the difficult part for me is the verb, いたはずだ. I want to say it is a very casual conjugation of the verb, いく, but I can only guess.