http://www.multitran.ru/c/m.exe?t=4884597_2_1&s1=confirmation of the non-acquisition of British citizenship

Letter of Confirmation of Attendance: Ivan Ivanovich Ivanov

This is to confirm that Ivan Ivanovich Ivanov, born [date of birth], is a student at (this school or [name of school]) and has been since [date of enrolment^†].

US: Sincerely yours
UK: (Yours sincerely or Yours faithfully^‡),

[Signature of sender]
[Name of sender]"

Something like that.

Note:
^{† US Eng = 'enrollment', UK Eng = 'enrolment'.
‡ In the UK, use 'Yours sincerely' if the letter's addressed to a named person, eg 'Dear Mr Smith'; use 'Yours faithfully' if it's addressed 'Dear Sir/Madam' or 'To Whom It May Concern'.}