Hydra's classroom was changed to an automatic recovery system without a health gauge.

In order not to spoil the tempo of the cover-action game, the damage system in Hydra's classroom was changed to an auto-healing system with no health gauge like in Gears of War, MGS, and CoD.
If no damage is taken for a certain period of time (depending on equipment and skills), an automatic recovery effect occurs.
If damage is received there, the recovery is canceled.
If you do not take any damage during the effect, your health will be fully recovered.