Above is a good article with some background. They say the parents became concerned about the boys not coming home as it got later in the day. They reached out to the coach (assistant coach actually) - and he found their bikes at the cave and went in after them.
Although that begs the question on why they weren’t found for another 9 days. Did the coach not tell anybody where he was going? Perhaps he knew of their plans and figured “I’ll just go and get them.”
It was the assistant coach in the cave. The head coach was alerted by the parents.
....”begs the question on why they weren’t found for another 9 days”....
Photos of the caves terrain and rescue would show you why....the flash flood waters were a great hinderence as the undercurrents were swift...once you view the inside of the cave, where they had to drill away stone, and how the search itself was filled with barriers etc. Does explain a lot.