300 x 5/100 = <<300*5/100=15>>15 senior students got a full merit scholarship.
300 x 10/100 = <<300*10/100=30>>30 senior students got a half merit scholarship.
So, a total of 15 + 30 = <<15+30=45>>45 students got scholarships.
Therefore, 300 - 45 = <<300-45=255>>255 senior students did not get any scholarships.
#### 255