First of all, the cultural context that needs to be understood in order to fully comprehend this sentence is that in Japan, it is not rare for companies to start a sideline business that is totally unrelated to its original line of business. Japanese language schools are often started by non-education-related companies.
The basic structure of this sentence is:
"A consists not only of B but also of C."
"Among A, there is B and additionally, there is also an increasing number of C."
Thus, the topic (A) is perfectly related to the rest of the sentence.
"Found among the companies that run the (Japanese language) schools (today), are education-related companies and additionally, an increasing number of companies involved in real estate, care for the aged, construction, etc."