Liveman was the first to feature students as rangers. It showed the value of true humanitarian education vs. the oppressive education of Volt.
Turboranger featured teenagers fighting against invaders. I think the emphasis of education happens mostly on Riki and Youhei as they learn along the way the importance of school. In the finale, the end song was "Graduation of Youth".
Fiveman was a team of not only siblings but also teachers in "New Town School". Gaku taught science, Ken taught sports, Kasumi taught Math, Fumiya taught languages and Remi taught music.
Megaranger was sort of like an upgrade on Turboranger for some. It carried out computers as the theme of the whole series. Like Turboranger, it featured teenagers fighting evil and Kenta Date/Mega Red was the slacker/leader who later learns to be responsible, in fact he became a teacher. The ending also had a graduation ceremony too!