Success in reform programs is defined by two factors - a prisoners understanding of a subject which you can not change and also a prisoners concentration on their studies, which is related to their needs. Understanding is specific per prisoner, some prisoners are as you say not academic enough to do well in basic/general education courses.
You've already identified that suppressed prisoners do worse in programs because they are less willing to learn. This means it is helpful to keep armed guards away from the classroom areas. Similarly, the more general sense of law and order there is within your prison the less punishment will be dished out (which adds to suppression). Keeping a safe, well guarded environment will keep the demand for weapons etc. low.
The remainder of prisoners needs must also be kept as low as possible to maximise a prisoners concentration:
- Keeping prisoners well fed
- Allowing time for prisoners to shower, exercise and use toilets
- Dealing with drugs and alcohol in the prison - intoxicated or detoxing prisoners will not concentrate on their studies well.
One way to minimise prisoners needs before taking them to courses is to allocate a 2 hour block of free time before the reform program is scheduled. Hopefully this gives prisoners freedom to reduce whichever needs are bothering them most before they have to sit down and learn.
The final factor, outside of the understanding and concentration values, is attendance. It goes without saying that if prisoners aren't in the classroom for their sessions, they wont stand much chance of passing. Prisoners might not turn up because they are in a foul mood, are locked in solitary/lockdown or are injured or unconscious.
By keeping your prison calm and safe by keeping your prisoners happy you will maximise your chances of having prisoners pass general education.