Why are privates school and public schools the same?

so i have been to both private schools and public schools. i went to public school from kinder garten to 3rd grade and then from 4th grade to the 11th grade i went to a private school in california and i am now 17 and in 12th grade and am currently attending a public school again. but my question was ..why is private school usually costly and has stricter requirements if they are very much alike if not the same... for example from 6th through 11th grade in the private school many people had drugs and got into fights and even some had intercourse on campus ...it was crazy! and it was very easy to deal druggs because the dealers would do it in the bathrooms and the security guards would only guard the front door the lunch corridor and the office hallway ..most of them just sat in the teahcers lounge awaiting deployment to a classroom or something ..it was nuts and i cannot believe my parent paid 2,000 a year for it! it seems like a rip off! ......so now i am attending a public school it is pretty decent compared to private school ..here at this public high school there arent really drugs other then the weed smuggled by the potheads ..the fights only happen 2 or 3 times a year ....and there is many security at this school ..and the security ride ehicle around the campus and are buff tall guys ..unlike at private school ,whos security was skinny 40 -50 year old men in uniform. also there are alo of them walking around the campus all through out the school day ....everything seems better and people seem happier at this school...and the best thing is that its FREE! so i wanted to know why is it that private schools charge money if there is little to no difference.