learning - Can Sempai be used to address a student of the same year/age

I've been working on a script for a Visual Novel, in it one of the heroine's Sakura address's the main hero Talavaliau (Tala for short) as Sempai because she has a crush on him and can't bring herself to actually call him by his name even with honorifics but also as a sign of respect for his nature

i base this on a number of anime/manga/books/games i have seen where a younger girl addresses an older crush as sempai and in most cases their just a class year's difference or have a year gap between their age

i know that sempai is used as a form of respect for younger students to address older students however in my script, Tala ends up being reassigned into Sakura's class so they are in the same year in the school, Tala's and Sakua's age are the same but Tala's birthday falls on the same day as his foster sister (he no longer remembers his real birth date and uses the day he was saved/adopted) which is only 1 month before Sakura's

so i am wondering if Sempai can be used to address a student of the same year/age or if i should use another term which doesn't involve using a person's name

Answer

It can refer simply to age, but especially in the context of school it will refer to someone in a grade above you. It's a similar thing in work or other organizations: your 先輩 are the ones who came first and outrank you. Outside of a strict hierarchy (like school or work) I think it just refers to the idea of someone being older, as in 彼は私の5年先輩です。 (He is five years my senior.) It may also be worth noting that dictionary entries tend to roll up the idea of simply being born first into the same definition as seniority in academia or in arts, so this may be more of a reference to someone who is more wise or knowledgeable rather than just being older. For example I don't think you really refer to elderly people as senpai.

Technically your classmate is your 同輩 (douhai), although as istrasci (and formerly snailboat) reminds us this term is not the most common, and instead 同級生 or other more common terms would be used (emphasis that this describes the relationship, not the term you would use to talk to them). These people would be addressed with kun/chan/san/whatever else. When in doubt, just stick within the established hierarchy. As a (pro?)noun used to address people, though, you have senpai but not much else.