No, it is not common to use 先生 (xiānsheng) in place of 老师 (lǎoshī) when referring to a teacher in a school setting. In a school setting, 老师 (lǎoshī) is the appropriate term to use. 先生 (xiānsheng) is generally used to address a male teacher in old times.

However, in modern times, 先生 (xiānsheng) can also be used as a polite form of address for any adult male, not just teachers or scholars. Its usage has expanded to address males in a general, respectful manner. So, while it traditionally referred to teachers and scholars, it's not limited to them in contemporary use.