It does sound like the groundskeeper is an employee of the city of Portland, as he was referred to as a turf-technician with the city of Portland. As such, he's probably not an employee of the school district. His job title suggests he mows the lawns at all city-owned property. So, if he's not an employee of the school, the school staff may not have a definite schedule for when the soccer field is mowed - only noting that the guy that mows the field is there when they see him mowing.
Kyron was reported missing later in the afternoon when he didn't get off the bus. The Hormans rushed to the school, and 911 was called. If I recall correctly, it was around 3:57pm when LE arrived. By that time, the school staff may have completely forgotten about the soccer field being mowed that day. If the groundskeeper is not a school employee, they would have no reason to keep track of him. Some may not have even known that the soccer field was mowed that day.
So, I can see the groundskeeper being overlooked.