If Xiaogang's injury does not constitute a minor injury, he can sue Yang for the crime of provoking trouble. If it constitutes a minor injury or more, he can be investigated for intentional injury.
As for the other two suspects in this case, Guo and Cheng, who are under the age of 16, as long as they have not caused serious injuries, they cannot be investigated for criminal responsibility. Only administrative penalties can be imposed, such as detention fines, and civil compensation should be borne by parents and guardians.