The website of Hongkou District knows:
The school name Tongji means “people are on the same boat and help each other” in Chinese.
No idea how they get that whole expression in only two signs… According to dict.cn the first one (同) means like, similar, with, together, same, alike, the second one (济) aid, ferry, frugal. Hm.
And with the boat in mind even the Tongji logo makes sense: