Theta is REM sleep, and it's an awake alert and open state. If you have good teachers in school, students will access a theta state and will learn. They are attentive, alert and open and most importantly they feel safe. What we DO know is that the brain does NOT go to theta when it is scared or in hyper alert. When we are afraid, the amygdala kicks into action.
Unfortunately, one of the things that causes our hyperalert danger sentinel, the amygdala, (gatekeeper for the hippocampus), to fire, is UNSAFE. If the classroom is deemed unsafe by the child, the hippocampus begins to respond and potentiate the wrong information. This means a child can go through an entire day of school and not recall anything except who got in trouble, and go through an entire year with a teacher that scares them, and they won't really have learned anything.
But! 50 years later that adult can still tell me minute details about that teacher from their elementary school days, the mean one, the scary one, the one that terrified them in some way. THAT information is retained in complete detail, and you can be sure that anyone that reminds their internal sentinel of that teacher, set of alarms. It doesn't always make sense to the adult, but it makes perfect sense to the amygdala-which has no sense of linear time.
The brain was online, it was just told to focus on the wrong thing. Right for survival. Wrong for learning.