Not counting SpongeBob, there are a total of five cadet fish.
List of cadets
The cadets who are seen include (from left to right):
A male purple fish
A female light blue/teal fish who is thin
A very short chubbyish male brownish-yellowish fish
A tall male dark yellow fish
A tall female light pink fish (who is about the same height as the male dark yellow fish)
Role in episode
The Milkshake Academy cadets are skilled, unlike SpongeBob. They are able to perform any task that Captain Frostymug tells them to do with ease.
They are first seen just outside the Milkshake Academy, jogging with large jugs of ice cream on their backs. When Frostymug singles SpongeBob out, asking what he is holding (which is an ice cream scooper), SpongeBob says it is an "ice cream spatula," and all five of the cadets laugh at him. Frostymug then tells a short story on how he is going to turn them into milkshake machines, and tells them to get to their milkshake stations.
All five cadets are successful at blending up a milkshake, but not SpongeBob, as he does not know where the control panels are. Frostymug then has the purple cadet tell SpongeBob where the real controls are. SpongeBob then blends up a milkshake, although it ends up smashing into Frostymug's eye.
The cadets then follow Frostymug into the Shake Simulator room, where he decides to put SpongeBob in the machine. Frostymug sets it to the highest setting, much to the horror of some of the cadets. SpongeBob ends up having fun in the machine, greatly upsetting Frostymug. Frostymug then loses it and asks the cadets if they have any tips to teach SpongeBob. The shortest cadet implies that Frostymug is better off "teaching a hunk of coral." All five cadets laugh and SpongeBob storms off, saying that he would show them.
The cadets are then put through a series of tests, including a cherry shooting range, and even volleyball. They all manage to complete their tasks, but SpongeBob is once again unsuccessful in doing this.
The Milkshake Academy cadets are seen for the last time at a milkshake ceremony as Captain Frostymug hands them all their milkshake licenses. They all leave the academy excited to be qualified milkshakers, but SpongeBob is left without one of his own (though he does receive one at the end).
Out of the five cadets, only three of them have speaking roles.
Captain Frostymug calls all the cadets "boys." However, two of the cadets are actually girls.