I have used this technique for years to help kids do their homework and study. Focus is one of the top problems that leads to incomplete work and declining grades. In my 20+ years of experience I have found that often times kids cannot sustain the full 25 minutes. For them, 15-20 minute work cycles and 1-3 minute break cycles seem to work best. There are many Pomodora timer apps. Forest is a great one for kids as it makes a game of the timing. Try to get one that allows you to change the time intervals. This is my #1 go to when changing work habits in kids. [click the picture above for more information]
My #2 go to for improvement. Humans need an average of 7-9 exposures to something in order to commit it to long term memory. "Not just a memory technique, repeating and reinforcing information you want to retain in long-term memory is a necessity. " (https://memorise.org/brain-articles/memory-repetition-reinforcement) Besides doing these one of these memorization techniques, also try adding movement. That way you can picture yourself doing something like a test. [click the picture above for more learning techniques]