update multiple table refrencing new table data

I have a table called ADSCHL which contains the school_code as Primary key and other two table as RGDEGR(common field as SCHOOl_code) and RGENRl( Original_school_code) which are refrencing the ADSCHL. if a school_code will be updated both the table RGDEGR (school_code) and RGERNL ( original_schoolcode) has to be updated as well. i Have been provided a new data that i have imported to SQL server using SSIS with table name as TESTCEP which has a column name school_code. I have been assigned a task to update the old school_code vale ( ADSCHL) with new school_code ( TESTCEP) and make sure the changes happen across all 3 tables. can someone please help me with this. i tried using Merge Update function but wasnt able to do it. it errored out.