As correctly answered by John T Smith, promotion pricing are for one year. So according to terms and conditions prices change after first 12 months of the purchase to the then-current standard price (i.e $29.99).
You may click on the below mentioned link to subscribe to a new discounted pricing for student and teachers:
With the above mentioned link you can even subscribe to one year prepaid plan in which you can pay for the complete year at one time, if you are eligible as a student or teacher.
Let us know if this is helpful,