18+ Student Oyster issues.. How to get discounted ticket?

I have a student 18+ Oyster and I have to travel from Watford Junction to Old Street. The normal ticket is �71.70 and I was told by my college that I can pay �10 for a student oyster and get 30% off, so I got the card and now whenever I go to Watford Junction they say I cannot get the discount there but I can get it online. However on the website it says I have to get it from a station, can anyone help?