A reference is required to point number 2 (1) of the Notification No.6/2005-ST dated 01.03.2005 on Exemption to Small Service Providers, as amended from time to time. The same is produced below for reference:
"2. The exemption contained in this notification shall apply subject to the following conditions, namely:-
(i) the provider of taxable service has the option not to avail the exemption contained in this notification and pay service tax on the taxable services provided by him and such option, once exercised in a financial year, shall not be withdrawn during the remaining part of such financial year;"
Where you have started charging Service Tax from the first bill, it amounts to exercise of option for non availment of above exemption notification. THe same could not be withdrawn for the remaining part of the year. Hence you should continue to charge & deposit service tax on your invoices.
But where the turnover of current year does not exceed Rs.10 lacs, you can claim above exemption next year, provided other conditions given in the notification are satisfied.