If I have a Raven Bloom Student and I play a spell and my opponent defys' the spell, will the Raven Bloom Student get +1 might?

No, your Ravenbloom Student will not gain +1 Might if your spell is countered by Defy.

According to Riftbound rules and official FAQs:

• Sequence of Events:
  1. You play a spell (it enters the chain).
  2. An opponent plays Defy in reaction to your spell.
  3. Defy resolves, countering your spell.
  4. Because the spell was countered, it fails to resolve.
  5. A card is only considered "played" for the purposes of triggered abilities like Ravenbloom Student's when it successfully resolves (Rule 412.1.b).

Since your spell never resolved, the condition "When you play a spell" is not met, and Ravenbloom Student's ability does not trigger. This is explicitly addressed in multiple official FAQs (e.g., FAQ #11082, #8945).