they r high schoolers dunno why all have come with the idea of middle schoolers ( maybe the uniform) but the earlier chapters shows clearly that they r in high school (from the descussion of course) they r even thinking about university ( i mean xixi and jian yi) in a near chapter whydo they think about they university theywillattend if they r just middle schoolers
i think U! should read again. yi was asked what "highschool" he will attend by a girl. and at the beginning it was CLEARLY said that yi disappeared right after highschool started .so all u see is either middleschool flashbacks or when we see them grown up its college time for xixi and work for momo.
that they are only middle schoolers, he tian especially is nothing like a middle schooler