Looking on the internet deeply has found these results:

I had an exam in English is the most popular phrase on the web.

I had an exam on English 10 results on the web	I had an exam in English 13 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
It doesn't look like this phrase is very popular! The search could not find any examples on the internet.	Mar 8, 2015 ... That being said I had an exam in English the other day and realised how terrible my english has gotten after I couldn't remember the word for ... 2015년 4월 3일 ... Today, I had an exam in English and science. 오늘은 영어와 과학 시험이 있었다. 2. I was so sleepy while the exam was going on. 시험이 진행 ... Mar 16, 2010 ... I recall a day in college [when] I had an exam in English and had some difficulty. He offered to help me translate texts. . . Between translations ...

I had an exam on English

10 results on the web

I had an exam in English

13 results on the web

More popular!

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.

Mar 8, 2015 ... That being said I had an exam in English the other day and realised how terrible my english has gotten after I couldn't remember the word for ...
2015년 4월 3일 ... Today, I had an exam in English and science. 오늘은 영어와 과학 시험이 있었다. 2. I was so sleepy while the exam was going on. 시험이 진행 ...
Mar 16, 2010 ... I recall a day in college [when] I had an exam in English and had some difficulty. He offered to help me translate texts. . . Between translations ...