If opponent plays watcher against my student who currently has 2 might, and I play a spell, does it stay at 1 might or go to 2?

Based on the rules and FAQs for Thousand-Tailed Watcher and Ravenbloom Student, your student will end up at 2 Might.

Ruling

When Thousand-Tailed Watcher reduces a unit's Might, it does not create a static "lock" at 1 Might. Instead, it "snapshots" the amount it reduced the unit's Might by at the time the effect resolves and applies that as a modifier for the rest of the turn. When your Ravenbloom Student buffs itself by playing a spell, that buff is added to its current Might value (which includes the snapshot modifier).

Sequence

1. Watcher Resolution: Your 2-Might Ravenbloom Student is affected by Thousand-Tailed Watcher. The ability calculates the reduction (2 Might - 3 Might = -1 Might). Because the Might cannot go below 1, the result is 1. The game "snapshots" this as a -1 Might modifier for the rest of the turn.
2. Spell Played: You play a spell.
3. Ability Trigger: Ravenbloom Student's ability triggers and adds +1 Might to its current Might value.
4. Final Calculation: 1 (current Might) + 1 (self-buff) = 2 Might.

This interpretation is consistent with several RiftJudge FAQs regarding Thousand-Tailed Watcher (e.g., FAQ #5538, #8027, #8931), which clarify that subsequent Might increases are applied normally to the modified value.