From intermediate level onwards private lessons are definitely the best way to go. This is because the Chinese skills of students at higher levels are usually very uneven. For example a HSK 4 student from Japan might be able to read a whole book, but struggling with basic conversations while a US expat who lived in China for a long time speaks at HSK 4 level but struggles to write his own name in Mandarin. Both would be classified at a similar level, however their actual abilities are completely different which means that class is almost always to easy for some and too hard for others at the same time. So from approximately HSK 4 onwards progress in group classes becomes very slow and we recommend signing up for individual classes from that level onwards.