Okay ...
There is mixed content warning which appears when the files with url http loaded over https .
Please make sure that you have provided https in your site url and home url also in wp-admin -> settings -> general .
Adding students with the help of emails is working fine at our end .
refer :
http://screencast-o-matic.com/watch/cDft1r1J4v
This may not working due to js errors in your site .
Please note that the old select box was too heavy and it loads all the values in selectbox at the page load which in some cases can crash the page .(coz there maybe hundereds or thousands of units,question in a site ) .
this is why we have revamped this and created an ajax search box which makes a vary little load on the server as the scope is bounded by a number of characters starting with .