I have 2 ipv6 network provider
network 1: ddbb5678::/48 gateway:ddbb5678::1
network 2: 240c1234::/48 gateway:240c1234::1
ddbb5678::2/64 @WAN1
240c1234::2/64 @WAN2
ddbb5678:1/64 @LAN 2
240c1234::1/64 @LAN 3
device:Mikrotik RB750 Gr3

I'd like mark packet from LAN 2 as school, letting packet marked as school default route to ddbb5678::1(gateway)
mark packet from LAN 3 as publicnetwork ,l, letting packet marked as publicnetwork default route to 240c1234::1(gateway)

I have set IPv6 Mangle to mark packet from LAN2 and LAN3.But route by routing mark function is unavailable in IPv6-Route,while in ipv4-Route,it is available.
How Can I let IPv6 Route by routing mark?

I think with Mikrotik, your only solution is to use two routers.

Unfortunately, IPv6 route marks are still unavailable - see this thread for more info - viewtopic.php?f=2&t=60155&hilit=ipv6+route+mark

It is unfortunate that the feature is postponed "because it needs the new routing engine" while in reality the
Linux kernel has this feature built-in (for IPv6 just as well as for IPv4) since ages.
It would not be so bad when that new routing engine actually existed (as a released feature), but now we
are stuck waiting for something that is postponed all the time.
Today our second fiber was installed and we have two IPv4 and two IPv6 subnets, but only for IPv4
it can be used for routing on our CCR1009. Pity.

but now we
are stuck waiting for something that is postponed all the time.

project unicorn?

project unicorn?

It reminds me all the time of project longhorn, the development that resulted in Windows Vista
after half of the promised features were ripped out at the last moment, and the product was a big fail.

One would hope that software companies would learn from eachother's mistakes, but no.

Yes, it looks like this feature isn't ever going to arrive, at least not in this decade, and we're likely to need to move away from Mikrotik as a result