That Crazy Creative Writing Teacher

I wrote this for Writing Exercise 5, the free write. What can I do to make this story more exciting? Should I add more to the beginning or end? Help me, my homiez

Once upon a time, there was an old 39 year old teacher. he was a weird dude but for some reason, his students liked him and decided to celebrate his birthday. It was a late October day full of various treats and snacks. One especially delicious snack was a cake brought in by the now 40 year old teacher. All the children loved it as the teacher figured they would. Little did the young students know that the cake was full of something very special. The teacher laughed wickedly as he watched the students eat second and even thirds of the cake. When the students got home that afternoon, they realized the true evilness behind the all black cake they had eaten second block that day. It was filled with a special ingredient that turned everyone who ate it, green from head to toe. Many of the students frantically called their doctors, not making the connection between the teacher and his delicious cake. As the evening carried on, the students became more and more green. They started sprouting leaves in various crevices on their body. By the next morning, all the students had become little plants. Their parents were sad at first but most of the students were annoying so the parents were glad to have gotten rid of them. It still remains a mystery, however, to the students left in NHSS, why that weird teacher keeps so many plants in his classroom.