Teach say alg 1 in a nice suburban school and 35 kids are not a problem. Teach a high school class where 80% of the kids are FAS, all the girls were raped at age 6, massive drug/alcohol problems, weapons, massive social dysfunction, and for the most part the 11th graders range from a 2nd to 8th grade level; 20 kids will eat you alive.
The teacher breaks the class into groups according to skill ability levels. So you use 4 different books and teach 4 different classes in one hour. This gives each group about 15 minutes of instruction time for an hour long session. Not enough time. Add in all the behavioral problems that will arise in some schools and you get an idea why small class size is important.
You always have 3-4 kids with tough emotional problems. The kids that will be institutionalized at 18; and sometimes these kids are very bright students. Now if we could only provide the one-on-one attention they require. The type of kids that urinate in the classroom, have violent outbursts, exhibit destructive tendencies, have psychological problems, and generally disrupt the classroom instruction. It seems as though each year there are more of these type of students in every reg ed classsroom; comes from public law 94-142 ad least restrictive environment; from the FEDS.
No simple answers mainly because all our ed problems begin in the home. Its a reflection of how as a people we are raising our kids.