Highschool sneak in-----?

I have a public high school extremely close to my house but I do not attend there. My high school that I attend has a week more of summer break than the public school near me does, so I thought it might be fun to pretend to be a freshman and sneak into the public school for a bit on the school's first day. (btw I am a sopre so there isn't’t a huge physical difference between me and freshmen) And I will spend most of my time in the lunchroom and avoid clrooms. However, I am nervous about getting caught leaving the school or entering late. What will they do if they figure out I’m not a student there? Could there be any serious consequences of my exploration? (Serious answers only please)