In grade school, you will probably experience both kinds of learning groups as you move through school. There are benefits and problems with both.
It is easier on the teacher to just assign by calling names, pulling out names from a can…., irrespective of gender. Most teachers don’t use this method very often for assigning groups, however, because some students in the classroom obviously should not be paired with certain other students. It doesn’t take long to figure that out once school begins.
Therefore, teachers will often think carefully about group assignments. While girls tend to learn better when segregated from boys, that is not always feasible in a mixed gender classroom. In a mixed gender classroom where groups are same-sex, there will be groups of girls and groups of boys. They boy groups will often take on that pack mentality boys have and do a lot of goofing off. The teacher winds up having to help the boys more because of it. Girls will often sit quietly (or semi-quietly) and do their work with only a little help. However, when they do need help, they may not be able to get it because the teacher is focused on keeping order and helping the other groups.
When you have co-ed groups, what often happens is that the boys will try to get the girls to do most of the work, and the girls will let them. I have seen this phenomenon many, many times in a classroom. You must have rules and watch that this is not going on. Otherwise, it is sometimes good to have a male and female perspective depending on what the group was put together for in the first place.
Going co-ed doesn’t have a lot of impact on the really young grades, but as they grow older, problems arise. As a teacher I can tell you that no matter which way you do it, you will have some kind of problem. So what you do is prepare for possible problems ahead of time, set rules and enforce them and keep your eyes open. Don’t let the boys suck you in too easily. Many times they can do it, they just don’t want to. Giving them too much attention then causes them to repeat whatever behavior got the teacher to help them and becomes a growing cycle.
So, if you can trade with another teacher so that one of you has boy groups and one of you has girl groups for certain learning group projects, go for it. If not, get to know your class and make your groups according to your class dynamic. Since most groups are short-lived, they can change and allow different students to work together over the year.