* UNICEF says 400,000 Syrians will need schooling in Lebanon
* Plan to open Lebanon schools 24 hours to educate Syrians
* U.N. envoy Brown says $175 million needed for school plan
By Michelle Nichols
NEW YORK, Sept 23 (Reuters) - Pakistani education crusader Malala Yousafzai and other youth activists challenged world leaders on Monday to come up with $175 million to educate 400,000 ...