Is it possible to log out of 'mail' on the iPad?
As far as I know, this is not possible. iPad is not made to be a multi-user device. I am in a similar situation myself as I share the iPad with my wife but haven't yet fixed it. Some alternatives:


Set a different account as default account in Settings > Mail, Contacts, Calendars. It could even be a dummy account. At least it means email will not be sent accidentally from your address.
In Settings > Mail, Contacts, Calendars > [Your email account], you can turn off Mail temporarily. It does not provide any security though, anyone can just turn it on again.
If you want to guarantee security, you need to use something like Gmail in Safari and login and logout every time.