Today I lost a sizeable check made out to a school club. What should I do? Can anyone who finds this check use my acct number to withdraw funds?
Currently, the check has not gone through or been cashed yet.
Call the bank and ask them to cancel the check. You should know its sequence number, and that's the information which the bank needs to "stop payment".
They will probably charge you a modest fee for this service.
Ask them specifically if they need you to additionally make this request in writing - some banks apparently do, according to https://www.thebalance.com/stop-payment-315346.
And ask if the "stop-payment" request will need to be renewed after some period. Some banks don't maintain these requests beyond six months or so.