Basic Trope: A teacher who speaks and teaches in a manner geared towards kindergarteners or first-graders, but actually teaches much older students.
Straight: Ms. Smith teaches her 14-year olds that blue + yellow = green.
Exaggerated: Dr. Smith teaches her college or grad students that blue + yellow = green.
Downplayed: Ms. Smith teaches her 8-year olds that blue + yellow = green.
The class is a remedial class that needs to go at a much slower pace than the other classes.
Ms. Smith is an art teacher, and this bit of revision is an introduction to something a little more advanced, such as how to change the shade of the green paint.
The school relies on standardized testing to get funding and so forth, and apparently "blue + yellow = green" is something important to the test. In other words, Ms. Smith is The Dumbledore; she doesn't appear to know what she's doing, but in fact she does.
Ms. Smith is a new teacher at a staunch Assimilation Academy, and her teaching style is her way of defying authority. (YMMV as to whether she's going about it the right way.)
Ms. Smith only has experience teaching in preschool and/or kindergarten and has no prior experience with teaching older children.
Inverted: Ms. Smith teaches her 14-year olds quantum physics.
Ms. Smith speaks in a childish manner, but she teaches something appropriate to the students' level.
"Now, when the Big Bad Multiplication Wolf came to the Bracket House and found the Three Little Terms all hiding inside, he huffed, and he puffed, and he multiplied them all individually!"
Double Subverted: Once her students catch on to what Ms. Smith is slipping into her child-like explanations... they find that it's still way below their level.
Parodied: Every class at Trope State University is like this!
Zig-Zagged: Ms. Smith's high, girlish voice has the class dreading that they'll be treated like kindergarteners, but then she teaches something appropriate to their grade level with a song. Then she gives a comprehensive test on the subject and is not afraid to tell you where you went wrong. But if you've shown all your work but still got an answer wrong, you get a gold star just for trying!
Ms. Smith teaches at an appropriate level.
Ms. Smith challenges her students by giving more advanced lessons.
Lampshaded: "Maybe I should teach kindergarten next year."
Invoked: Ms. Smith, a kindergarten teacher, got laid off. She is desperate for work, and a nearby high school needs a teacher. Since she only knows how to teach kindergarten, well... Old habits die hard.
Exploited: Mr. Grumpy takes stress leave, and arranges for Ms. Smith to take his classes for a while so his students will want him back.
Defied: Ms. Smith takes the job at the high school, but she soon adapts to teach high schoolers appropriately.
Discussed: "I think Ms. Smith would be happier teaching kindergarten."
Conversed: "Certainly her students would."
Deconstructed: Talking down to students makes them lose interest; it's something they already know, and their intelligence is being insulted. If they lose interest, they won't learn anything. If standardized tests are important, the students won't know how to answer the questions, and the school could lose funding, or the students won't get into the high school or college they want.
Ms. Smith notices her students losing interest, and reevaluates her teaching style, so that she's teaching lessons appropriate to the grade level of her students.
Alternatively, Ms. Smith decides that if she can't do that, she's going to take a new job teaching kindergarten, preschool, or first grade. The students then learn something appropriate to their level, and she gets to teach her way.
Played For Laughs: The students begin reversing the roles and teach Ms. Smith. Competently, even.