Learn Russian: Мы дружим со школы. - We have been friends since school.

Russian has a specific verb for "to be friends" which is дружить. It is much more natural to use this verb here than to say мы друзья (we are friends), especially when talking about how long the friendship has lasted.

Why is дружим in the present tense when the English translation says "have been"?

When an action started in the past and is still continuing into the present, Russian uses the present tense. English uses the present perfect ("have been"), but in Russian, because you are still friends right now, you use the present tense дружим.

Why is the preposition со used here instead of the usual с?

The preposition с changes to со before words starting with certain consonant clusters, especially those beginning with с, з, ш, or ж. Because школы starts with the consonants шк, adding the о makes it easier to pronounce.

What case is школы in, and what does it mean here?

It is in the genitive case. When the preposition с (or со) is followed by the genitive case, it can mean "from" a place or "since" a certain time. Here, со школы means "since school."