Time controlled login rules

Hi,

I administer a school blog used by a class of 8 and 9 year olds. Each child has their own username and password and can publish posts in their lessons.

In order to maintain an element of security and appropriateness, I want to limit the time that users can log into the site so that they’re only using it within school hours and not at home on an evening.

I’ve been searching for solutions and plugins for a few days to help with this but haven’t found anything. Does anybody know plugin or other way to restrict user logon to set time limits (similar to Windows parental controls function)?