Should i tell my school about this **IMPORTANT**?

alright I'm in highschool and after school while i was waiting for my parents to pick me up. I saw a 10th grader pull out a gun like a real gun but he didn't point it to anyone he was showing it to his friends so i just blew it off my shoulder now I'm rethinking about it.Should i tell the school tomorrow or just let it go? I don't know this kid name.