At time, it has been discovered that old, recurring meetings will occasionally not display as Busy to Outlook, allowing Students to accidentially schedule over them.
In the event that this occurs, you will need to recreate the event and delete the previous ones.
Note: If this is a persistent issue, check the initiator of the meeting. If the person scheduling the meeting is a licensed CRM user who has not done Calendar Syncing, this can create sync errors for all events for any users who are invited to attend.