Niagara College has raised concerns over the validity of the English-language tests taken by students from India.
The Toronto Star reports a probe by the college found "inconsistencies" in language proficiency.
It has asked 428 applicants to undergo a second English test or risk losing their offer of admission.
The popular International English Language Testing System is accepted by most academic institutions.
Steve Hudson, the vice-president of academic and learner services at Niagara College, says the school launched the investigation this fall after the number of first-year international students at risk academically rose to 300 from an average of 150 in other years
An Immigration Department spokesperson said officials are looking into the matter.