Normally you can have only users with unique email addresses in your database. That means you may not have two or more users (teachers or students) which share the same email address in your student database.
Sometimes however it is necessary to be able to create several student profiles with the same email address. This is the case when, for instance, different family members share the same email address but need to have separate profiles in your database.
In this case you have to disable the unique email address requirement on your configuration and create teachers or students with existing email addresses, you can do so as described below:
Log into your admin profile. Visit Account Settings -> Look & Feel and make sure to "Allow registration of users with duplicate email addresses"
Finally click Apply.