For the filling, in 10-inch skillet, heat 1 ½ teaspoons oil over medium heat until shimmering, about 2 minutes (oil should be hot but not smoking). Add zucchini, pepper, and 1/8 teaspoon salt and cook, stirring occasionally with wooden spoon, until wilted, about 1 minute. Turn off heat. 

Place colander in sink. Transfer zucchini to colander (ask an adult for help). Let cool completely, about 10 minutes. Transfer zucchini to medium bowl. Add mozzarella and jalapenos and stir to combine. 

For the dough, in second medium bowl, combine masa harina, water, and ¾ teaspoon salt. Stir with clean wooden spoon until combined. Use your hands to knead mixture until dough forms, about 1 minute. (If many large cracks form when dough is squeezed, stir in more water, 1 teaspoon at a time, until dough no longer cracks when pressed.) 

Divide the dough into 6 equal portions, roll into balls, and place on counter. Cover dough balls with plastic wrap.

Shape and fill pupusas: Working with 1 dough ball at a time, use your hands to shape dough into 4-inch circle, about ½ inch thick. Scoop 2 tablespoons filling into center of dough, pull sides over filling, and pinch at top. Roll into smooth ball. Repeat with remaining dough and filling. Lightly grease your hands with oil. Working with 1 filled dough ball at a time, gently flatten dough with your hands into 4 ½-inch circle, about ½ inch thick. Place on rimmed baking sheet and cover with plastic wrap. Repeat flattening with remaining filled dough balls. 

Adjust oven rack to middle position and heat oven to 200°F. Set cooling rack in second rimmed baking sheet. 

Heat 12-inch cast-iron skillet over medium-low heat until hot, about 3 minutes. Add 1 teaspoon oil to skillet. Hold skillet handle with oven mitts and swirl to coat skillet with oil (ask an adult for help). 

Add 3 pupusas to skillet and cook until well browned on first side, 4 to 6 minutes. Use spatula to flip pupusas. Cook until well browned on second side, 4 to 6 minutes. Transfer to rack set in rimmed baking sheet and place in the oven to keep warm.  

Repeat cooking with remaining oil and pupusas. Use oven mitts to remove baking sheet from oven and place on second cooling rack (ask an adult for help). Serve.