You will need $3.10.
School Yard
Talk to Cindy and agree to become her boyfriend. She will ask you to stick gum in Lily's hair. Say: Sure > Like what? > Yeah. > Okay.
Talk to the teacher about Buggs stealing your money. Agree to get into a fight with him. Pester Buggs until he starts to beat you up; then click Call for Teacher.
Morning Time
Talk to Lily, then stick the gum in her hair.
Talk to Cindy. Say: She cried > Okay then > Okay then. After that, play house with Cindy. Say: Yes dear > No honey, I'm just tired > Is that a breathalyzer? > I'll take the test > Be a father? > We can work through this > Yes I do > Keep it > No problem > Okay > Sure.
Lunch Time
Next, you need to get yourself and Cindy something to eat. You can get a helping of slop, or by eating one of Nugget's nuggets (obtainable from one of the cubbies if you give the teacher a pill). For Cindy, you can buy her a salad from Monty or a "Biscut Ball" from the janitor. Since she doesn't know what vegan means, giving her a Biscut Ball will still continue the mission.
Once you have the food, talk to Cindy. Eat the slop/nugget, and then give her the salad/ball. After that, she'll bribe the lunch lady, so that you'll can go to the janitor's closet. When in the closet, get the bucket of blood. You must leave the closet as soon as the bell rings (when you have 1 apple left. Using the last apple will cause the janitor to enter the closet and kill you). Return to the lunch room. The lunch lady will say that your class has already headed to the recess, and you will join them.
Recess
Once at recess, you must go to Cindy and talk to her. Once she asks, you must climb the monkey bars and dump the blood on Lily. She will give you her flower. After that, you will go to show and tell.
Show and Tell
Share with the class the flower that Cindy gave you. She'll let you keep the flower, earning you a special item and completing the mission.
If Buggs is present during show and tell, he will beat you up (kill you) for showing off the flower. This is why it's necessary to expel him in the beginning of the mission. (or remove him from show and tell in any other way).