Translation:Let's stop in front of the school.
I am a native English speaker, and "Let's stop in front of school." sounds fine to me. That's usually how we talk (at least in America) when referring to our own school, we'll just say "school", but if it was an unfamiliar school that no one in the group went to, then it's more likely someone would say "the school." But I don't think it's grammatically incorrect to leave out "the", or in any case, people talk that way regardless.
You would say that if it isn't a school you normally go to. For example if you work in a factory but there is an event on at a local school you might say "I'm going to the school today", whereas if you told your coworker "I'm going to school today" it would sound a bit weird, like you will be going to do a day of lessons.
止める is transitive, which means it needs an object. "I stop the car" for example.
止まる is intransitive, it does not take an object: "The car stops".
In this sentence there is no object, so we use 止まる. There are many of these transitive-intransitive verbs, 開く、開ける , 落ちる, 落とす etc. so it is important to learn the differences!
Is there any way to know this is referring to location rather than time? I put "Let's stop before school", thinking this a time reference. Would you not refer to school (the time you are at school) in Japanese as 学校? Or would you use に instead of で to indicate time instead of place?
our is just a possible answer. we can assume that the speaker suggested that they along with the listener stop in front of the school, and we think of the school as theirs, thus "let's stop in front of our school." that said, nothing should be wrong assuming the school is the speaker's or the listener's only, and saying my or your school. or even his or her if a third person had been mentioned before.