Here are some thoughts on how you can improve your game:
- The jump is a little bit floatty, which makes it both weird to control and easy to land wherever you want to go. Make it snappier by adding more gravity for example, and maybe reduce air control.
- Adding sounds and music helps a lot to make a game feels more finished. I guess you didn't have time, but if you want to continue working on this I think you should definitely start with that.
- I collected items but had no clue what they were supposed to do for me. I guess the sword was supposed to help me fight the big guy at the end, but I couldn't find how. A small line like the one you wrote as tutorial at the beginning would have helped.
- I died to that boss without any idea how it happened. I guess I took damage, but nothing on the screen told me that. There are simple ways to communicate this, like flashing the character red or playing a sound.
This is still very impressive for someone so young, I hope you'll continue on this path and make great games in the future!