Learn Turkish: İster keman çal ister davul, bu okul herkes için. - Whether you play the violin or the drum, this school is for everyone.

It translates to "whether ... or ...". It is used to show that the result will be the same no matter which option happens. It actually comes from the verb istemek (to want), giving the feeling of "whether you want this or want that."

Why is the verb çal used like a command here?

In the ister ... ister ... structure, the action is often written in the imperative (command) form. Even though çal looks like it means "Play!", in this pattern it simply translates to "whether you play". You can also use the conditional form for the same meaning.

Why is there no verb after davul?

To avoid repeating the same word, Turkish often drops the second verb if it is identical to the first one. Saying ister keman çal ister davul çal is grammatically correct, but leaving out the second çal is much more natural, just like in English.

Should there be a "to be" ending like -dir at the very end of the sentence?

You could say herkes içindir, which sounds a bit more formal or like a general fact. However, it is completely normal to leave the "to be" ending off in everyday conversational Turkish. Bu okul herkes için is a perfectly complete sentence on its own.