I don't agree with the opinion that says students will 'check each other out' or flirt in class. Putting boys with girls is a great way to teach them not to do these things. It's better for students to make mistakes and be corrected in school rather than later in life, and the proper distance between individuals of opposite sexes is one of those things. As a side note, this kind of seating is more often than not a good way of separating friends, which means they'll talk less in class, i.e. less distractions. It's more likely for two boys to be homies than a boy and a girl to be a couple.
The only way for children to develop proper social behavior is to interact with other children. You cannot separate girls and boys in classes and then expect them to act socially acceptable when they finally come together. Also by keeping them separate you make it appear more sinful to be together. This makes hooking up more exciting.
In primary school, as a girl there was nothing worse than sitting next to boys in class. If the class didn't behave everyone would have to sit boy, girl, boy, girl. This meant that you couldn't sit next to your friends, but rather some icky boy! I feel that in primary school boys and girls should definitely be allowed to sit together in class, perhaps middle school too, highschool on the other hand would depend on the teacher and the school.
Boys and girls are opposing genders, and besides that they will try to show off to each other and try to be cool, girls can connect better with other girls. Girls feel more comfortable with wit own gender, and it has been proved that girls are more open to admitting mistakes and fixing their work if it is their friend they are working with.
The students may try checking each other out or flirting. The school environment should be a place for learning and learning ONLY. Students may not focus as more if they have someone of the opposite gender sitting next to them. They may try to act cool and show off by showing that they're too cool for school and oppose the teacher or not do their work.