SCH heals

[Brightamethyst]

and I've seen the tank HP get frighteningly low on large pulls before going back up..

That's by design. Excogitation, the biggest heal SCHs have, only works if the target is below 50% health. You HAVE to wait until the tank is at low health before you use it.

Also, with healing it's sometimes hard to tell the difference between panic and skill. Letting a tank drop really low might be a sign of a bad healer or a mistake, but it could also be a really good healer who knows exactly how much healing is needed when.