Greg responds...

I was probably speaking somewhat loosely.

I guess if I'm really going to break down the metaphor, here's how I'd do it at the START of Season Two:

SENIOR CLASS
B-01 Nightwing
B-02 Aqualad
B-03 Kid Flash
B-04 Superboy
B-05 Miss Martian
B-06 Red Arrow
B-07 Artemis
B-08 Zatanna
B-09 Rocket
B-10 Tempest
B-11 Aquagirl

JUNIOR CLASS
B-12 Troia
B-13 Robin
B-14 Lieutenant Marvel
B-15 Sergeant Marvel
B-16 Batgirl
B-17 Bumblebee

SOPHOMORE CLASS
B-18 Lagoon Boy

FRESHMAN CLASS
B-19 Beast Boy
B-20 Robin
B-21 Wonder Girl
B-22 Blue Beetle

By the end of Season Two, it's more like this:

POST-GRADS
B-02 Aqualad
B-04 Superboy
B-05 Miss Martian
B-07 Tigress

SENIORS
B-16 Batgirl
B-17 Bumblebee

JUNIORS
B-18 Lagoon Boy
B-24 Guardian*

SOPHOMORES
B-19 Beast Boy
B-20 Robin
B-21 Wonder Girl
B-22 Blue Beetle

FRESHMEN
B-23 Impulse
B-26 Static

*B-24 Guardian's a bit of a special case, since he's been around awhile, but has only been an active hero for a short time. So think of him as a transfer student.
B-25 Arsenal would be a freshman if he came back.

But keep in mind it's JUST a metaphor. Don't take it too literally.