Is it ....A Flock ... of Hippos?

Clearly it is  (collective noun) of hippos swimming across the River Oose, near St Ives.

But what is the correct collective noun for swimming hippos?

My brain says "A School Of..."  but I'm not 100% sure.

Bonus - US troops testing a panzerfaust on a knocked out Tiger II.

Bonus 2 -