I'm looking for a little help here as I'm getting contradictory advice at the moment. We're currently stuck at the planning stage of an Office 365 implementation.

Currently we have an onsite Exchange 2010 server for staff and no pupil email provision.
We want to set up Office 365 with the same domain name as our email server for staff and pupils.
We're not ready right now to migrate staff email off our server into Office 365.

I am being told that our MX records can be set up such that if our Exchange server is set at highest priority and our 365 domain a lower priority email will try the first one and if the account doesn't exist there it will then try the second one. I didn't think this was how MX records worked, I thought it was an all or nothing proposition, either the server is available to accept the email or it wasn't.

Also if anyone has any suggestions as to how we could run this sort of set up (pupils in 365, staff email in local Exchange but staff still having accounts in 365) I'd be grateful.

At the moment I'm thinking the only real way forward is to leave the MX records pointing to our Exchange server so we can at least get going on setting up Office 365 with affecting staff email, and then migrate staff email to 365 over the summer and decommission our 2010 server.