Effective ways to block out background conversation?

Basically I have a phobia and I've let my doctor and teacher know however, during one of my lessons my teacher has made me aware that next week (for 3 hours) we'll be discussing the topic (which happens to be my phobia). Since it's totally okay for me to leave if it's really stressing me out she's advised me to just try and see how it goes at first as these 3 hours contain content that's pretty important to our exam BUT she says of course if I really can't stand to discuss or hear any conversation about it she's totally fine with me blocking my ears and just ignoring the lesson whilst that particular thing is discussed and mentioned that if it's stressing me out way too much I can just ive her a signal and leave the room for a bit.

The thing is, whilst it's all very well blocking my ears there's only so much noise I can drown out and I can't sit there and sing to myself because people will notice, despite the fact I'm pretty much behind everybody and my friends on my table are aware of my phobia so I have support and everything but I'd like to know if there are any effective ways of blocking out background conversations without the use of earphones (as I'm obviously not allowed them in class). I know some noise will get through but if there's a good way of ignoring it to the point where I can't tell what's being said or where I cn just simply forget it completely as soon as it has been said.

Just to end, my teacher is in no way forcing me to take lessons (I'm in sixth form, over 16, so I'm of no legal obligation to attend them), she's allowed me to dip out of the last hour of the lesson today and just set me some work on the lesson that I don't mind revising - she always makes sure I hve enough for the exam and just takes out the bits I can't handle. Despite this, I can't miss too many lessons as I have a whole exam on this so it's really just these 3 hours that I need to get through if I can. Any suggestions would be much appreciated, thanks in advance!