It looks like you have the Mathjax filter installed. The default for render the TeX code initially with a mode called 'Preview HTML' which displays the mathematics as quickly as possible with standard fonts and sizing. Once that is complete it gradually replaces the expressions with other formats depending on browser and user preferences.
Each user has the option to control this by right clicking on an expression to open a context menu. The output format can be changed from the Math Settings -> Math renderer submenu. You can change to Preview HTML if you like, but that is not usually considered the best. If you want to disable the preview unset Fast Preview. This will however not change what students see. They can change the display themselves. If you want to change that, you can with the config, but you need to read the Mathjax docs to do that.
Another display option is to enable the older TeX notation filter. This will render expressions as image files so that everyone sees the same image (although not necessarily the best). You can also enable both filters. The TeX notation filter will usually show first replaced a little later by the MathJax results.