Andover’s short answer writing prompts say to write less than 150 words. I’ve written 174 for one but I don’t really want to make it shorter… Do I have to trim it or will it be okay? Thanks!
You have to trim it. They have the guidelines for a reason and so it’s bad form to go over the limit when you were given clear instruction not to.
Start by getting rid of adjectives and adverbs 
Andover has asked you to write 150 words or less for a reason. They truly want to see how well you can say everything that you need to in a few number of words if you went over, all it’s telling the admissions staff is that you are incapable of expressing your thoughts well and concisely.
Also, Andover has, I believe, about 3,000 applicants each year and in order to get through reading every one of them, they need to be concise.
You should defiantly stay within the word limit. I had this problem a few times too.
Most will not let you add more ( they limit you by characters).