Why is that seminarian wearing a full surplice? I thought only priests wore such.
There is no such thing as a "full surplice" or a "priest only surplice". Some are lighter, some are heavier, some have lace, some are completely solid. But fundamentally, a surplice is a surplice. The design and materials vary, but the fanciest lace surplice could be used by a 7 year old altar boy, and the simplest cotton surplice could be used by the priest while hearing confessions.
Maybe you're confusing it with another white garment -- an "alb" which goes down to the floor. These are only worn by priests (or clerics at least) in the Traditional rite.