“不要走” emphasize the action “走”，no matter you go away or departure, you have to walk ; but “不要离开” is departure from somewhere, you can walk away or fly away or offline. For example, 不要离开座位，can not say 不要走开座位。

你好！In the phrase“不要走‘，“走” has two meanings. The first one is walk. eg"不要走那条路，太脏了。You can not use"离开"for the fist meaning. The second one equals "离开（leave）"，so they have something similar. But they also have something different. "不要走"can not be added an object and it is an oral expression while "不要离开"can be added an object like someone or someplace. eg"不要离开我/这里/学校" And it is relatively a written language. So you should write "请大家不要离开教室'，it is a formal situation. I hope my reply can benifit you.

不要走 = don't go 不要离开 = don't leave

in the example you discribed , i suppose "不要离开" will be better in written form. It's more formal than the other.