Pleading not guilty at arraignment is not lying in court. Its SOP to pave the way for some type of deal. The school will understand this. You don't stand a chance of being acquitted, but that's not what the issue is. You expect to be punished and have learned your lesson (I hope). But the process is now about making sure the punishment isn't too onerous.
Before you enter a plea, you will be assigned an atty and have a chance to chat with them. Go by what they advise.
