Go to http://www.wbsed.gov.in/ > Click on NOTICE > Click on Item No 23 that reads Orders for Conversion of Non govt Aided schools into Govt Sponsored schools > Select District from the drop-down list > finally, select your school and submit
The order of Conversion particularly for your school will be generated. Download it.
N.B. : If you cannot find the name of your school there, you may also check Item No 6 instead of 23.