Cancelling a dance class after several students hurt themselves is an example of which risk management strategy?

Avoid strategy as the threat/risk is directly related to identified threats and these sources of threats are cancelled

Explanation:
Risk management is defined as: 
The identification , analysis, control and possibly elimination of any undesired hazards or risks arising from a certain situation or activity

There are four risk management strategies which are:
1- Avoid: To eliminate the risk by totally avoiding the situation/activity
2- Mitigate: To try to reduce the expected hazards of the situation/activity
3- Accept: To accept the hazards and take your chances
4- Transfer: To outsource the risk (as in case of insurance)

In case of cancelling the dance class, we can note that the risks are directly related to identified threats and the threats are totally cancelled in order to avoid those risks. Therefore, the best strategy describing this situation is "avoid strategy"

Hope this helps :)