How to summon multiple mobs at once in Minecraft bedrock? I think that's what Minecraft EDU is. (Zombies specifically)
There is no easy way to spawn a bunch of mobs at the exact same time. However, if you want to spawn a bunch of mobs all at once you can place a tower of command blocks and use
/fill to fill the tower with redstone blocks to trigger all of them at once.
You can build the tower in this fashion. It is one block per floor and extensible to the height limit.
_C_ CRC _C_ Key: _: Air/Doesn't Matter (as long as not redstone-triggering) C: Command Block (with summon command of choice, you can use Ctrl+MidClick to pick block with data) R: Redstone Block to be placed with /fill