Dear Mr xxxxxx or Dear Mrs xxxx
Dear Sir/Madam or Principal
I have received the relevant documentation and application form with regard to applying for (a place in you school). or whatever position you have applied for.
I regret, that due to unforseen circumstances, I wish to cancel my request/ registration.
Please accept my apologies for any inconvenience I may have caused.
Yours sincerely ( If you are writing to a named person) or Yours faithfully if you are starting off by saying Dear Sir/Madam
Hope this is what you need.
Student or Learner