Which category belongs to kayasth caste?

In Bengal:

In the time of Lakhsman Sen, he had to ban the then brambhins-Chatterjee, Banerjee, Bhattacharjee due to their ill tendency to take "Utkoch" from people for rising them from lower caste to higher caste. Then few brambhins came from Mithila with some servant-Ghosh, Bose, Mitra & Dutta. Those servants r Kayastha and they r belonging to Sudra as per Manu.