I now have more info.  School probably not defunct.  Name:
MOUNTFTTCHET HIGH SCHOOL, FOREST HALL ROAD, STANSTED, ESSEX CM24
8TZ,ENGLAND. Ann Whitecross attended this school from Dec 17, 1968 to
July 21, 1973.  There are no copies of examination certificates
because the EXAMINATION BOARDS only issued these to candidates and did
not send a copy to the school.  Further, the EXAMINATION BOARDS no
longer exist.  What legal docs can she provide a college in USA so
that she may attain a BA equivalent degree from a MOntessori
Institute?  She has a letter from the Mountfttchet High School in
England dated 9th April 1997 and signed by the a Headmaster, D. J.
Williams, stating that she attended the school for the years noted
above.

I don't know the details, but it seems that it is quite possible to
attend some colleges without having a high school diploma or
equivalent documents.

http://www.womeningovernment.org/highered/NYT_CollegewithoutHS.pdf.pdf

I never finished high school. When I entered college at age 16, my
scores on the SAT and ACT were sufficient to get me in.