What does 社会 mean in this context?
社会力量办学；
  


社会 VS 国家 or 政府


  2.此为公交专用车道，社会车辆不得驶入;


社会 means any 车辆 that not from 公交部门


  3.走出校门，走进社会？


Here is the typical usage of 社会.

Most of organizations was run by government before 改革开放, these organizations  are usually called 机关, 单位 or 组织 etc...

If a man does not work in that kind of organizations, he lives in 社会, he is not 组织中的人.

For example, 社会人 is an interesting words, in some situation it means they do not have a formal job, they have complex relations with different kind of people... it is very complex, I did not prepare for this term.

And 学校 is opposite of 社会 in China. 

社会 is not only society in China, it has some negative meanings from history periods.