Rarely do authors think about these kind of things.
The big ice spike on her head looks like roughly 1 kg of ice.
With the values for water content in the air from this table: at 30 °C (since the chapter is in summer) with roughly 30 g/m³, that would mean roughly 30 m³ at full water saturation would be needed to make this one ice shard.
Which is possible considering the class room has probably something like (6x10x3 = 180 m³) as volume. But the picture at end with the buckets full of ice are most likely dragon magic or they would have needed to cool down and dehumify enough air to probably change the local weather.
PrincessKadath@ani.social 4 days ago
I thought about the moisture in the air myself, but in the end opted out for the ‘dragon magic’ as you more aptly defined it.