I have to say, this is really a toss up depending on where you go. My personal experience while my wife was dealing with a MH crisis is that ERs are so overwhelmed most of the time that they cannot (or maybe do not) take the time to help or diagnose a MH crisis. If you're lucky the hospital social worker will be around when you go, but again that's a crap shoot.

Not saying going to the ER is bad advice, but the reality of the situation can be a lot different than what you're hoping for.

+1 direct communication with your professors and being frank about your situation can be a huge thing. Most (likely not all) professors should be kind/understanding enough to give you some leeway, or make alternate provisions for classwork, and it all starts with communication.