Here is how the Lunch Lady Ghost arrives and the food fight in the cafeteria goes in Crash, Thomas and Ryan Meet Danny Phantom.
(we then see Ryan Tokisaki stayed behind)
Ryan Tokisaki: Now to find something to eat.
(But before he can, he sees the Fenton Ghost Portal activate)
Ryan Tokisaki: What is that?
Matau T. Monkey: The portal just opened on its own. Meaning something is coming from the other end.
(They hide as the Lunch Lady Ghost comes out)
Lunch lady Ghost: Ooh. Someone changed the menu.
(She turns intangable and phases through the roof)
Ryan Tokisaki: We better warn the others.
Matau T. Monkey: He's right.
(They head out)
(at the cafeteria)
Danny Fenton: So, like, you're a secret organization that no one should know about?
Ryan F-Freeman: Don't know.