German Teachers of English and Native Speakers

My children--who have been talking to me, Mom, from Manhattan, all their lives, and listening to me reading them everybody from Harry Potter to Rick Riordan--got 2s on their report cards in English (the American equivalent is a B.) They bring home stories about their English class almost daily that have us all in stitches or in open-mouthed disbelief . . . and now their report cards tell me how much the teacher resents their command of the language. Yesterday's sample: My daughter's Nigerian classmate, who, like her, grew up in an English-speaking home, pronounces "three" as "tree."
"No, that's wrong!" said their teacher. "The word is pronounced 'Sssssreeeee!'" Now, I could post a You-Tube video tailored to native speakers of German who want to know where to place their tongues in order to pronounce the "th" sound in "three." But the point is the teacher ought to be doing that herself. She also ought to have some idea of Global English, and the wide range of pronunciations that appear on CNN and BBC. Nobody's doing "RP" or "received pronunciation" anymore except . . . well, the Nigerian father of one of my kids' classmates who said he wanted to learn American black English because people were telling him his British English sounded "affected." 
I did tell my daughter that if she wanted to get in even more trouble with her teacher she might toss around linguistic terms like "interdental fricative." 
But my kids never wanted trouble with the teacher. They just can't help rolling their eyes when she says the word "thrice" doesn't exist. 
Not that these gals speak English badly. They're teachers, after all! They'd get along fine in airports and restaurants anywhere in an English-speaking country. 
But they're also Germans. They like the feeling of authority when they believe it's due. They like to be sure of themselves, and they like the sound of the pronouncement, American English sounds like chewing gum, even though they all must know that just ain't so. I've heard 'em get all their pronouns wrong ("Children, look TO my mouth," pronounced "mouse") and I've heard 'em pronounce "lettuce" as "Let-oooose" (to rhyme with moose.) I admit to speaking German much, much worse than they speak English. But I do like to learn, and I do use You-Tube, and I do go to other Internet sites to at least try to get my pronunciations correct. I sure do wish the English teachers of Germany would do that. I also wish they'd enjoy students who already speak the language . . . even the ones who speak it much better than they do. Or especially. You know, teach, you'd have more fun that way.