In Japan, is it odd for a 20 year old student to flirt with an 18 year old HS senior?

Friend of mines writing a manga and one HS senior whose 18 and a female protag whose a 20 year old uni student have a small tease such as they releasize they have some stuff in common and he thinks she's cute and she thinks he's sorta cool
Sure its just a manga but I want to ensure he's not including weird stuff so how would this be seen overall?

No it's not odd.