What if an eighth grader and ninth grader like each other?

If an 8th grader and a 9th grader like each other, it should be no problem. As long as the two feel the same way about each other, it could be a relationship like any other. It might, however, be difficult to spend very much time with the person, since they don't share classes or even the same school building (in most cases). Still, outside activities can be arranged, and it shouldn't be much of an issue.