An answer from the daily mail...
“It was apparently a local initiation ceremony for boys to run to the end of a cave tunnel and write their name on the wall before running back.
While they were all inside, however, a flash flood sealed off the exit forcing the boys to head further into the mountain.
Armed with a single functioning torch, they managed to reach the spot where they have subsequently been discovered.”
http://www.dailymail.co.uk/news/article-5915565/How-terrified-Thai-boys-there.html
Thank you.