"Let's stop in front of the school."
-ましょう is more like "Let us," so it's not really a command. (link for more: https://jlptsensei.com/learn-japanese-grammar/%E3%81%BE%E3%81%97%E3%82%87%E3%81%86-mashou-meaning/)
If you were telling someone else to stop, you would say 学校の前で止まってください。
Basically if there was a は after it the school would be the subject of the sentence. (The actual subject is an implied 私たち, which would be followed by は, but both are usually dropped in regular speech, especially with ましょう.)
では would be more like "There is a ___ in front of the school." (Although I would say "には", personally)
It's a different verb, meaning "to stop something," basically: https://jisho.org/word/%E6%AD%A2%E3%82%81%E3%82%8B
There are a lot of verb pairs like that in Japanese. Similar to verbs that require a direct object in English vs those that don't.