In West Roxbury, a powerhouse in the exam-school admission race, the overall number of seventh-grade applicants receiving admission offers dramatically declined from 133 last year to 69 this year under a temporary policy that capped admission by ZIP code. The tougher competition meant students needed an A minus to get in.By contrast in Mattapan, a neighborhood where families typically have less means to hire admission consultants, the total number of seventh-grade applicants getting in more than doubled to 51 this year, with some candidates who had a B-minus average securing a coveted acceptance letter from one of the exam schools.