What is law re: writing about a college in a fictional book or script?

Trademark infringement is using a trademark to indicate the source of goods or services in a way that creates a likelihood of confusion with some else's trademarked goods or services. Talking about something in a book is not likely going to be a trademark infringement. However, you should consult one on one with an attorney about what you are intending to do so that you can be sure that none of your intended uses are an infringement. It is far better to do so up front than have to change things after the fact.

A lot will depend on what you say including the entire context. While it might not be trademark infringement if the name is used in a non-trademark sense, the could be other contexts that could be disparaging or create a false designation of origin. As pointed out, you need to discuss the details with a lawyer familiar with relevant areas of law and don't seek legal opinions online.

There's no bright line rule about what's permissible and what isn't. "Oscar Wao" is partially set in Rutgers and "The Marriage Plot" is set at Brown. (And Bard didn't sue Steely Dan for writing "My Old School.") You need to be careful not to over-incorporate IP that doesn't belong to you into your work, or make use of mascots or logos or other protective trademarks. If you're uncertain whether your text is infringing, consult an IP attorney for review.

Excellent question, I would recommend being very carful about naming a collage and disparaging it. If you are casting the school in a bad light, then, by all means use a fictitious name. I would also recommend that you do not display school's logos or other protected items. Once you prepare your manuscript, you should have an experienced Trademark/IP attorney review and clear the manuscript for trademark infringement and exposure to claims of disparagement.
I would recommend that you ask an experienced copyright and trademark attorney to provide you with their opinion in confidence. To do so, I suggest that you first contact an experienced Copyright and Trademark/ IP attorney who can advise you in confidence. Many IP firms offer an initial free conference by telephone, video conference or in person if you are available locally and would be happy to speak with you. Call and speak with an experienced Trademark and Copyright attorney who can assist you.

You are free to write about anything or anyone you want. When you choose to write about real people or real entities the onus is on you to be sure that whatever you are saying is not painting them in a false light. If so, then you expose yourself to claims for defamation (libel or slander).
It is always best to get signed releases from parties that you do reference after they have had a chance to review your work. This greatly mitigates chances of law suits later and certainly helps put publishers at ease. But it is not required.
If you plan on taking great liberties in your work regards to the school, the way it conducts its business, etc. then I would use a fictional school unless you have every reason to believe what you write is the truth. If however the school is not really the subject and is more incidental and background then you can use any real school you want to paint the scene.
I suggest that you consult with a lawyer in private and discuss your objectives in more detail. You can start by calling around to several for a free phone consultation, get some insights then pick the best fit to work with.
Best regards,
Frank
Natoli-Lapin, LLC