Japanese high-school is only 3 years. There is no sophomore, just use first, second, and third year. Especially if you are going to translate kouhai to junior, since junior in high school in the US (and probably other English-speaking countries) is 11th grade which is older than sophomore which is 10th grade. So a Japanese 2nd year high schooler would be an 11th grader in US. So the way it is now in your translation, it makes Umeko sound like she is somehow an upperclassmen and underclassmen at the same time.
If the issue is you don't know how the Japanese educational system is set up, then here it is:
Elementary/primary/grade school is 6 years. Middle school or junior high is 3 years. High school is also 3 years. University is 4 years. Elementary and middle school are compulsory (required to be attended by law and enforced by truancy officers) and high school is optional (not required by law).