Why "Él está en la escuela" and not just "Él está en escuela"?

Why "Él está en la escuela" and not just "Él está en escuela"?
 

Why is it proper to say, "He is at school" as "El está en la escuela" in Spanish, instead of just "El está en escuela"?

Especially since "El está en casa" is like English in that it is just "He is at home".

Note that 'en casa' means 'at home', even though 'casa' is the word for house, not home. This is a set phrase.
'In the house' is said 'en la casa'.

'En escuela' isn't a set phrase. You must say 'en la escuela'.

So would I always use "la/el/las/los" after prepositional phrases unless it is a set phrase?

Is there any way to know when to use set phrases always, or is it just by memory?

Set phrases need to be memorized.

'En casa' isn't actually a prepositional phrase, by the way. It's an adverb of location (adverbio de lugar).

Yes, an article appears after the preposition and prior to the object. The preposition, the article and the prepositional object make up the prepositional phrase.

Thank-you very much!