The parents sometimes arrive with kids who are 12, 13, 14 and they use the schools for a babysitting service while they work in the fields. This is especially true for kids who speak only their native tongue which is not Spanish but rather Mixteco.
It`s simple to track; one just goes to the school website and looks for Mixteco-speakers in the 8th grade and 9th grade and there are quite a few but in the 10th grade, there are no Mixteco speakers. They did not learn to speak English. They dropped out and went to work picking strawberries.