I like that I get credit for all good advice, but noojens made that postmirage wrote:A few schools use an open house as an interview for applicants. They will observe for a couple of days those applicants that have been short-listed and then make their final offers. So unless you're sure that you don't want to attend said school, make sure that you understand the purpose of the open house. But like grae said, if you have better offers then please go ahead and reject the school.
So if they made you an offer, your offer is good until April 15th. If they did not make an offer but gave you the opportunity to visit, they are using it as an interview. However, noojens is absolutely right and if you know for sure that you are not going to attend a certain school, withdraw your application if a decision is still pending or send them an email politely declining their offer of admission.