The only way the ghosting for 1 year would have made sense is if Misa mistakenly believed that Jun and Kawai started going out, but then she'd have to give up on him and that didn't happen at all; not to mention she'd have noticed they weren't going out.
1 month would be more believable, but the mangaka probably thought 1 year would be more dramatic (and they probably wanted to skip it forward to high school). At least they've made up now, but they'll have a year's worth of catching up and patching up to do.