I concur with Ann.
It’s hard to help without knowing what this assessment is for in more detail and the context of the student.
Is the purpose diagnostic or summative? To identify what they do or don’t know?!
If you Google ‘English Proficiency Test’ then a variety of links come up including British Council, IELTS (commonly used for university applications) and SELT (for citizenship applications). If a test is standardised/accredited, then there is likely to be a fee.
For which exams is the school trying to improve the student’s access?
For GCSEs and A Level I would have thought the diagnostic materials/guidance provided by the BELL Foundation would be more appropriate for a Y12 student, assuming (as Ann says) that for some reason time in the country isn’t sufficient and evidence of need is required.
Best wishes
Di Leedham.