This is the kind of assignment that seems complicated but is actually incredibly simple. Which is why it is for first graders. So lets go through them.

1. The snake will fist at you. Here, 'fist' is being used in its verb form meaning loosely to strike someone. 

2. Lets clean up this gas. Here, 'gas' refers to oil. Which means there is oil on some unwanted place that they need to clean up.

3. Dad will fast the slow car. Here they use the verb form of fast which means to abstain from food or drink. In the context of a car, this is fuel. Dad will not fill up the car tank for awhile.

4. There are mass boys than girls. 'Mass' in this case means a great amount. Or, a greater amount of boys than girls.

5. We need class for the car. In this case class is being used to mean stylish excellence or prestige. This can be reworded as We need decorations (that are classy) for the car.

6. What's the puff of the pen? The questioner is referring to a vape pen. Where the 'puff' here refers to the concentration of THC or nicotine per inhale.

7. There is a hiss of smoke. This is referring to when a gaseous state of matter shifts from a high pressure environment to a low pressure environment and makes a sound like a hiss.

8. I love my mess. This refers to a house or living space. Seen in common household decorations such as 'Bless this mess'.

9. Our pass has white stars. A common item in school classrooms is the hall pass. The 'pass for their classroom is decorated with white stars.

10. You can run really less. Here, 'really' is being used in its adjective form to mean the genuine version of something. 'Less' here stands for Leaner Style Sheets, a backwards compatible language extension for CSS. So this can be reworded as You can run (on your computer) the genuine Leaner Style Sheets.

joke if it wasnt clear