Easily switch group of contacts to a newly created contact subtype
Create a profile (Administer > Customize Data and Screens > Profiles) that includes the Contact field "Contact Subtype."
Bring up your desired group of individuals (whether through a search, group listing, etc.).
Select the appropriate individuals and use the "Batch Update via Profile" action.
On the next screen, select your Profile that contains the "Contact Subtype" field and click Continue.
On the Batch Update screen, select the Student subtype for the top individual, then click the Copy icon next to the column label "Contact Subtype."  That will copy the Student subtype selection all the way down the list. Scroll down to the bottom of the page and click "Update Contacts".


You're all set!

Note: the limit for Batch Update via Profile is 100 contacts, so if your number is greater than 100, you'll have to do Steps 3-5 multiple times.