В конце концов, давайте обратимся к официальным источникам. Вот, например, Vancouver School Board что говорит:
http://www.vsb.bc.ca/combined-split-classes
What is a combined class?
A combined or split class refers to a class that is made up of students from two or more grades. Combined classes occur most frequently in elementary schools and are the combination of students in two different grades with one teacher teaching both curricula.
For example, a school might have a grade 2/grade 3 class combination or a grade 6/grade 7 class combination. At
the secondary schools, combined classes may occur, for example a Spanish 9/10 combination or a Ceramics 10/11/12 combination.
It is important to know that students in combined classes are not held back to the level of the younger children nor are the children in the lower grade expected to do work beyond their abilities. The prescribed learning outcomes are grade appropriate for all students, whether or not they are in a combined class.
Why do Schools create combined classes?
There are several reasons why schools create these classes. When school staff look to organize their students, that is to place the students in their classes with teachers, they have to balance the educational needs of their students with the staffing they receive from Human Resources as well as with legislated contractual considerations.
A school calculates enrolment. This is then verified by the Human Resources division of the Vancouver Board of Education. Based on this figure, teaching staff (both enrolling and nonenrolling) is allocated. The Principal, working with the Staff Committee of the school, then constructs classes based on these factors and the educational interests of each student. In many instances, combined classes result.
How do Schools decide which students to place in combined classes?
In an effort to ensure all students are placed in appropriately balanced classrooms, school staff (teachers, Principals and Vice-Principals) use such criteria as age, range of ability, special learning needs, gender, social groupings, and support staff recommendation.
Also, just as they do when allocating students to non-combined classes, the Principal and Vice-Principal work together with the classroom teachers to best match the learning styles and needs of the student. In many schools parents are given the opportunity in June to submit their preference for a learning situation for their child for the next school year. All these factors are considered before allocating classes for students.
Вообще, там много букв, пройдите по ссылке, если интересно.
. учительница сказала, что часть материала теряется в любом случае 
ну что поделать, только заниматься дополнительно.
чему тут следовать. Я просто привела факт, который опровергает теорию вашей соседки.
Про католические школы - я пас.