Aw I sympathise - I CRAWLED to work during my PGCE!! Tell your mentor how shit you are feeling in advance and get some lessons planned as back up to show him/her - they will be able to throw something together much quicker and easier than you in any case, so even if it is not brilliant, you won't be giving them massive amounts of work.
Remember the class is still the teacher's responsibility - they can easily take over if you are ill.
However, I would not have listened to this advice myself
(although bloody well should have!! Was ridiculous)so if you do drag yourself in, plan activities which require them to do most of the talking (card sorts/group work/silent reading (if it works!). Telling the class (even via tsk cards at the door) that they are going to learn something and then 'teach' or present it to each other could save you voice a bit as well.
Develop a silence/attention technique that doesn't require using your voice and use this all the time anyway - drama 'hand in the air' or the arsey clear your throat, fold your arms at the top of the class and looked bored (though not hostile) until they catch on (then smile and and 'thank you' or whatever) all work well. Depends on age/behaviour of class, though.
Overall, as long as you show massive willing to the dept and your mentor then you are allowed to be sick!! It is only if you swan in and out, or phone in at 8.45 that people start to roll their eyes.
Sorry, loooooong response! And sleep sleep sleep berrocca berrocca berrocca until you are an NQT!
Good luck x