Upi Patel, Bernie Enlander and Kathy Brooks founded Forvus in 1985 to analyse data for private and public firms.
It is now contracted to compile the A-level, GCSE and key stage 2 and 3 tables each year as well as working with the Teacher Training Agency, Customs and Excise and the Housing Corporation.
The business, which has a turnover of pound;3m and employs 20 people at its Grade II-listed headquarters in Clapham, south London, will retain the current staff and management.
Managing director Mr Patel, who lists his hobbies on the company's website as looking worried and playing snooker badly, said the deal will enable the company to grow faster.
He said: "It enables us to go for, and hopefully win, larger contracts.
There is a trend to consolidation within central government. It is offering larger contracts which are often multi-disciplinary, needing four or five sets of skills. By being part of a larger group we provide a more complete service."
The company has contracts with the Department for Education and Skills, compiling all the performance tables.
Steve Bollingbroke, education services director of RM and now the chairman of Forvus, said: "One of the things that we liked about Forvus was its fabulous management team.
"It has built up a wonderful relationship with its customers, all of whom speak very highly of the company. If you're buying a business, that is what we look for."