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先生がかわりにしてあげます？
I came across this sentence and I cant seem to understand why が is used here. I'm roughly interpreting this as: "shall I substitute for the teacher?/shall I do x in place of the teacher?" But I'm having trouble understanding why が is used here instead of の since I feel like I've seen this same structure used with の before and I've only seen の-が being interchangeable in relative clauses.
This 先生 is a simple subject marker. 先生 serves as the subject of する. This sentence may be easier for you to understand if the word order is changed a bit:
かわりに、先生がしてあげます？
Instead, will I (=Sensei) do it for you/them?
I believe this sentence was said by the 先生. In other words, 先生 is used as a first-person pronoun, which is very common in school.
かわりに ("instead") is an adverb that is used alone here. It may be "instead of you/them" or "in place of you/them" depending on the previous context (but not "instead of the teacher"; that would be 先生の代わりに).
