Professional Children's School was founded in 1914 by Jane Harris Hall and Jean Greer Robinson to support children working in the industry. In 1956 the school was moved to the 7-story building on West 60th street where it stands today.
I know very little about the Cowsill children's time there, other than occasionally hearing one of them mention going there. I'm not even sure when the Cowsills moved to NY, but since they signed with Joda there it was probably in the 1965-1966 time frame.
The Cowsill kids are not listed on PCS's website as alumni, so they may not have attended very long. According to a June 1969 newspaper article, they were still attending PCS in NY in 1969, but that doesn't seem likely as they moved to California in September 1968.