Your students can create a dynamic DNS host when go to http://www.dyndns.com and create a own DNS host like
"student1.dyndns.info" . With the program dyndnsupdater (he can download from www.dyndns.com)your student can automatically change his IP address by dyndns.
Then you can connect over student1.dyndns.info as IP-address to your student.
your students must have a RADMIN Server licence(42$)