I'm a supply teacher at the moment so unless it's a class/school I know well I don't know what methods the school use or what that class use. You often also have a mixture of children using more formal methods and then children using methods like number lines, especially if you have a wide range of abilities - I've been in Y2 classes where there's children using the decomposition method for subtraction! What I normally do in this situation is give the children some time on whiteboards to work out the problem - whilst they're doing this move around the class and look at the method(s) used and give a bit of support to those who might need it. You might find at this point the children use one method or there's a mixture like I described earlier. Then I would ask one child to show how they worked it out, getting them to talk through their method. Then I ask if anyone worked out the answer a different way, that way you're covering the variety of methods and showing they're all reach the same answer (if done correctly!).