You realise you guys can walk around uni anytime... no-one would know you weren't a uni student... no-one would care.
Just don't come in your school uniform, otherwise you'll look like a wanker.
Open Days are useful in the sense that they expose you to information and options that you probably haven't even thought about yet.
They put ideas on the table for you to toss around, and often open days really help you decide whether you like a university or not.