When I have a Ravenbloom Student on a battlefield and play a hidden card on the battlefield, does she get a buff or only when the hidden spell gets "played" as in revealed?

Ravenbloom Student gets +1 Might only when you play the hidden spell — that is, when you flip it up from facedown and resolve it on a later turn. Hiding it initially (paying [A] to put it facedown) is a separate action — it does not count as playing the spell and does not trigger Ravenbloom Student.

To be precise:

• Hiding a spell (placing it facedown at a Battlefield) — does not trigger Ravenbloom Student.
• Playing from hidden (flipping it up on a future turn, resolving its effect) — does trigger Ravenbloom Student, since playing a card from hidden still counts as playing it (FAQ #1301, #3691).