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Please help me correct them, thanks.

Last Monday was (our) Teachers Day. Many pupils were very excited to celebrate (it). They came early to school by 7 a.m. Some helped to sweep the floor while the other / others mop the floor, dusted the windows, arranged chairs and tables in a big circle nicely. Almost all the classrooms were decorated with paper stripes and balloons. They were having / had a small party for teachers.

Many pupils brought their home-make cookies and cakes to schoool. Some of them prepared plenty of mouth-watering food like fried chickens, cakes, cookies, sandwiches, nuggets and soft drainks. All food and drinks were / was arranged / were put nicely on the tables. The party began by 9 a.m. after having an important assembly.

When their teachers came, they sang songs and played games with pupils. After that, they presented some flowers and presents to show their appreciation to teachers. Many teachers attended the party in great joy. They were touched and thanked pupils for the celebrations. It was really an unforgettable and meaningful for teachers.
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I have underlined some problems:

Last Monday was (our) Teachers Day. Many pupils were very excited to celebrate (it). They came early to school by 7 a.m. Some helped to sweep the floor while the other / others mop the floor, dusted the windows, arranged chairs and tables in a big circle nicely. Almost all the classrooms were decorated with paper stripes and balloons. They were having / had a small party for teachers.

Many pupils brought their home-make cookies and cakes to schoool. Some of them prepared plenty of mouth-watering food like fried chickens, cakes, cookies, sandwiches, nuggets and soft drainks. All food and drinks were / was arranged / were put nicely on the tables. The party began by 9 a.m. after having an important assembly.

When their teachers came, they sang songs and played games with pupils. After that, they presented some flowers and presents to show their appreciation to teachers. Many teachers attended the party in great joy. They were touched and thanked pupils for the celebrations. It was really an unforgettable and meaningful for teachers.
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Last Monday was (our) Teachers Day. Many students were very excited to celebrate (it). They came to school by 7 a.m. Some helped to sweep the floor while the others mopped the floor, dusted the windows, arranged chairs and tables in a big circle. Almost all the classrooms were decorated with stripes paper and balloons. They had a small party for the teachers.

Many pupils brought their home-made cookies and cakes to school. Some of them prepared plenty of mouth-watering food like fried chicken, cakes, cookies, sandwiches, nugget and soft drinks. All food and drinks were arranged / were put nicely on the tables. The party began at 9 a.m. after the assembly.

When their teachers came, they sang songs and played games with students. After that, they presented some flowers and presents to show their appreciation to the teachers. Many teachers attended the party with great joy. They were touched and thanked students for the celebrations. It was really an unforgettable and meaningful day for teachers.
Better!

Last Monday was (our) Teachers Day. The pupils were very excited to celebrate (it). They came to school by 7 a.m. Some helped to sweep the floor while the others mopped the floor, washed the windows, and arranged chairs and tables in a big circle. Almost all the classrooms were decorated with paper strips and balloons. They had a small party for the teachers.

Many pupils brought their home-made cookies and cakes to school. Some of them prepared plenty of mouth-watering food like fried chicken, cakes, cookies, sandwiches, nugget ( What is this??) and soft drinks. All the food and drinks were arranged / were put nicely on the tables. The party began at 9 a.m. after the assembly.

When their teachers came, they sang songs and played games with the pupils. After that, they presented some flowers and presents to show their appreciation to the teachers. Many teachers happily attended the party. They were touched and thanked the students for the celebration. It was really an unforgettable and meaningful day for the teachers.