How to deal with an inappropriate greeting in an email?
Personally, I would address it in a friendly way, but one that makes it clear that you think it's a bit of an odd form of greeting. It doesn't sound like it was intended in an unfriendly or disrespectful way (and I definitely wouldn't characterise it as sexist), but it does sound inappropriately overfamiliar (it would be a bit like one of my students starting an email to me with "Dude! ...").

The easiest way to deal with it, to my mind, is just to send a fairly normal-sounding (but obviously more formal) email back, and mention it in passing in a friendly way at the end. For example:


  Dear &lt;name&gt;,
  
  Many thanks for your email - really glad to hear that you've been
  learning a lot from the course. It's been good to see the improvements
  in your essay structure since the start of the course - I'm hoping
  that you'll be able to solidify those improvements over the next 8
  weeks.
  
  Kind regards,
  
  &lt;name&gt;
  
  p.s. Just a friendly piece of advice - whilst I'm not generally too
  fussy about people addressing me formally, I think I should probably
  steer you away from things like "my dear lass", not least because some
  people if addressed that way might take it the wrong way and get
  offended. I'm fine with people calling me by my first name (if the OP
  actually is), but I think that's stretching things too far.