Translation:Every Monday, I practice guitar at school.
It's only slight wrong now. に can mark the: 1: indirect object 2: location of object 3: point in time (at/on/in for time in English) 4: Destination/Purpose of a motion verb (to, because) (5: agent of passive, causative, or causative passive) If you want to say: I practice guitar at the school, meaning you want to specify the location of the guitar practise (literally), it's で. If you want to say I talked to my Japanese friend, then it can be に, but と is even better (I spoke with...). I would consider "に" a bit rude, to be honest.
As far as I know, 毎週の月曜日 is acceptable, if not "official". As for 毎月曜日, it is the same as 毎週月曜日 in the sense that it's the same day every time, but it's meaning is different. In 毎週月曜日 the emphasis is on 毎週, and in 毎月曜日, the emphasis is on 月曜日. I would use 毎週の月曜日 if that is the subject -I wouldn't use a verb, an action, I would say, maybe: Every week's Monday is a short day.