Hmmm, interesting question. I think it probably depends on where it’s set. I could easily see a 14-year-old wanting to be in a relationship with a 16-year-old, but I’m having a hard time seeing it go the other way. I say this because 16-year-olds are usually juniors, so they’re busy cramming for the ACT/SAT (depending on where they live and assuming we’re talking about America). Their lives are very different. One is preparing to leave high school, while the other one has just arrived. The junior will be prepping for college and leaving home, while the freshman just came out of middle school, so there’s still going to be quite a bit of childishness leftover from that. Middle school really messes a lot of people up, but the junior would most likely have left that behind by the time they turned 16. Their friends are typically also 16 and also preparing to graduate.
Plus, they would be unlikely to be in any of the same classes together. The only situation where I could see them ending up in a class together is if the junior put off their foreign-language classes and the freshman started them right off the bat. Most high-school classes are fairly segregated by age group, so they won’t be in, say, English or math together. Other theoretical possibilities: science classes (depending on levels of ambition), early drama classes, and early art classes. A lot of classes have prerequisites, so it’s unlikely for the freshman to have taken the classes that would allow them to be in the same class as the junior.
In any case, they’re both in high school, and neither of them is about to graduate or anything, so it is most likely acceptable/appropriate.