"Let's attend school together."
Translation:학교에 같이 다녀요.
With motion verbs, when attached to locations, ~에서 is used to mean "from" and ~에 "to". E.g.
남자아이가 집에 학교에서 갑니다 - The boy goes home from school. (Where "home" means "to his house")
With non-motion verbs however, ~에 means more like where something is statically/passively happening and with action verbs ~에서 means something more like where the action verb itself actively happens. E.g.
나는 서울에 사요 - I live in Seoul. (So 서울 is where the "living" is happening regardless of action)
나는 학교에서 운동해요 - I exercise at school. (So 학교 is where I exercise, but that location could change or I could stop exercising)
Note that this means ~에서 can mean both "from" and where an action is done but context should clarify which one it is.
As for why it's not ~에서 in this example, the verb is 다니다 which is not a motion verb and is also not an active verb as it means more like "regularly go". Since you can "regularly go" somewhere without actually being there all the time and regardless of action, ~에 is the right particle and I don't think it actually makes any sense to use ~에서 for 다니다 since you can be attending school but not be at school at any given time.
Disclaimer: I'm no expert but this is how I made sense of the tips and hints and Ash-Fred's helpful comment here: https://forum.duolingo.com/comment/24359128
Please correct me if I'm wrong :)
"Let's attend school together." is a proposition but "학교에 같이 다녀요." is not. "학교에 같이 다녀요." would translate back into English to something like "We attend school together.", right?
I think "학교에 같이 다니자." or more naturally "학교에 같이 가자." are better translations of "Let's attend school together."