Although a creature put onto the battlefield this way is attacking, it was never declared as an attacking creature. Abilities that trigger whenever a creature attacks won't trigger when that creature enters attacking.

You can't activate an ability that advances a Class to a particular level if that Class is already that level or higher. For example, if a Class is already level 2, you can't activate the ability that advances it to level 2 again.

Class cards have multiple text box sections. Other than the top section, each section contains a class level bar that includes a cost and a level number. Each represents an activated ability with the listed cost that advances the Class to the level listed in that bar.

Spells can only be cast for their sneak costs any time you could play an instant during the declare blockers step on your turn (after your opponent has decided whether to block).

If a creature spell's sneak cost was paid, the creature it becomes enters tapped and attacking the same player, planeswalker, or battle as the creature that was returned to its owner's hand to pay its sneak cost. This is a rule specific to sneak; in other cases, when a creature is put onto the battlefield attacking, that creature's controller chooses which player, planeswalker, or battle it's attacking.

Casting creature spells from your graveyard using their sneak abilities with the permission granted by Ninja Teen's first level 3 ability doesn't change when you can cast them. You'll only be able to cast them this way during the declare blockers step on your turn when you could play an instant, and you'll still have to pay the sneak cost, including returning an unblocked attacking creature you control to its owner's hand.

Gaining a level won't remove abilities that a Class had at a previous level.

There's no restriction on how many Class permanents you can control, whether they're the same or different classes. Each Class permanent tracks its own level separately.

An ability that advances a Class to a higher level is a normal activated ability. It uses the stack and can be responded to.

If a creature card in your graveyard has multiple sneak abilities, you can cast it from your graveyard using any of those sneak abilities with the permission granted by Ninja Teen's second level 3 ability.

Each Class starts at level 1 and has the abilities in the top section of its text box. Once a Class advances to level 2 or level 3, it has all of the abilities listed in the associated section of its text box as well.