Panel 1: As Elly drives Mike and Gordo to the dance, we find ourselves watching The Human Doorstop scowling and angrily thought-bubbling "Why do we have to go to this stupid dance? I'd rather stay home and watch TV!" (Note: I know that he's thought-bubbling in hickspeak because Lynn wants to put on airs. I don't so I'm going to use proper English.)
The answer to his stupid whining is that Lynn wants to set up a ridiculous rescue fantasy that comes crashing to Earth and two more years of Mike jerking Martha around like a total fucking greaseball (and also two years of Elly shitting bricks about him selfishly getting Miss Dangerous Body Language pregnant).
Panel 2: He hates the nice outfit Elly picked out for him, he hates having his hair combed and he especially hates being driven there in that stupid car with the mother who seems to live to embarrass him in front of his friends.
(Since he thinks she proved that she didn't care what the scruffy nitwits he mistakenly calls his friends think when she sang in cafes long before he was conceived of and also conceived, his belief that she gets up every day plotting the ruin of his imaginary street cred is what Cracked.com calls "pants on head stupid.")
Panel 3: Since Elly got there on time, Mike thought-bubbles "GROAN!!!" because he and Gordon are going to be the first ones there. I thought-bubble "Groan" because we're about to be reminded that Lynn thinks that two kids getting in the party mood via drinking Thunderbird Wine is a lesser offence than geting stoned or going to second base.
Panel 4: Since Elly is poor at reading body language, she can look at the back of the heads of the two boys she's wishing "Have a good time!" to and not realize that the slouching means that both of them are scowling in theatrical rage.
Summary: The 'joke' is neither boy thinks he'll have a good time because Mike's fear of embarrassment and ridicule and inability to understand people messed up his love life and Gordon has to watch 'his' girl dance with a boy she likes.