Can a hidden Fox-Fire be played when there are no valid targets on the battlefield to trigger Ravenbloom Student's ability?

Ruling: Yes, Fox-Fire can be played with zero targets because it specifies "up to" any number of units with combined might 4 or less, meaning zero is a valid choice. This allows you to trigger effects like Ravenbloom Student's ability that care about playing spells.

Nuances:

• If there are legal targets available (including your own units, since Fox-Fire doesn't specify enemy units only), you must select at least one target unless you choose zero as part of the "up to" targeting.
• You cannot reveal a hidden card without playing it - there is no game action to simply reveal and trash a hidden spell to manipulate graveyard count without actually playing it.
• The "any number" clause in Fox-Fire applies to targeting, not resolution, so the zero-target option must be chosen during the targeting phase.