I asked a similar question on Baidu before. Others recommended two places, Chongwen and Haidian, which are both very good. I was near Haidian, so I went to the "Qin Shi Art and Culture Center" near Jimen Bridge in Haidian. The vocal music teachers there have been judges in major competitions and have taught vocal music for many years, so I signed up for several classes to try. After the first class, my high voice improved, and I'm still studying there. It's good. You can search "Jimenqiao Vocal Music Training" on the Internet to find their enrollment information.