Why are so many kids not passing?

Ever been that kid in the classroom, that is a having a ton of trouble with something? But every time you try to get the teachers attention they barely have any time for you? These days we have so many students shoved into one classroom that about 5-6 out a 15 student class aren't passing like they should. Teachers are becoming less available to students it seems like even though they are right there in front of us? It's like some teachers just pick and choose...