You should speak English more in class.何不可说成You should speak

You should speak more English in class.意思是在课上说更多的英语,指的是英语的内容要更多；
You should speak English more in class,意思是在课上多说英语,不管每次说的内容多不多,但是要多说,指的是说英语这个动作的频率及次数.