Four causes, maybe more but the four I think had the biggest impact:
1. Government funds = Government control
2. Greed and rise of teachers Unions, a powerful socialist/secularist organization that gets power through control of pay.
3. Drugs, drugs and more drugs so teachers do not have to manage classrooms.
4. Teachers teach to the lowest common denominator to prevent students from feeling bad. Better to keep every student dumb than hurt some kids feelings about themselves.
OK, next you have to ask yourself, (and answer) who allows such teachers and drugs to be in our classrooms?