Setting aside what the age difference is, which ranges from six to 1 year's difference depending on their individual year's (it never says what years they are) and whether middle school means Jr high, can we take a moment to appreciate how silly our female lead is?
In the previous chapter she had a very haughty bearing. A dignified presence. As befits a vampire who is older than they appear. This chapter, not so much. She felt much more like a Jr high student. Turns out, according to the devil boy, it's because she is just a kid. She's not a vampire. She's got a vampire bat soul.
Even though she actually is a supernatural entity, she still manages to be a chuuni. It's adorable.