I uttered these sentences in the class."First of all I will read a story for you people. Side by side I will give you the story map and you will do this activity in pairs."Is this utterance grammatically correct, if not why not?
Strictly speaking, I would not say that your sentences are grammatically incorrect, however, their punctuation may be improved. They could be written as follows:
First of all, I will read a story for you people. Side by side, I will give you the story map, and you will do this activity in pairs.
'First of all' and 'Side by side' are introductory phrases, i.e. their purpose is to set the context for the main independent clauses that follow them. Most writers recommend that such phrases should be followed by a comma. However, some writers say that this is not always necessary, especially for short introductory phrases. Such writers would say that the use of commas in these situations is a matter of style, with the decision to use them or not being best left to each writer. I suggest that you Google 'introductory phrases', read several of the articles that are presented, and then decide for yourself how you will punctuate such phrases in the future. I generally tend to use use commas in such situations, as I think it makes it easier to understand what is written.
The comma between the words 'map' and 'and' is a different matter. In this case the word 'and' joins two independent clauses. The two clauses:
I will give you the story map
you will do this activity in pairs
are independent clauses because they are able to stand by themselves as separate sentences. When two independent clauses are joined together using a coordinating conjunction (e.g. and, but, yet, for, so, etc.), that coordinating conjunction should be preceded by a comma.
My main objection to your sentences is that they are logically inconsistent. If you start out saying, 'First of all', then the reader is lead to expect that a second step will follow, and possibly additional steps after that. However, the use of the phrase 'side by side' implies that the two actions will be occurring simultaneously, not in a stepwise progression.
If you intended these activities to take place in a stepwise manner, you could have said something like:
First of all, I will read a story to you. Then, working in pairs, fill in the story map that I will give you.
If you intended these activities to take place simultaneously, you could have said something like:
I will read a story to you. As I do so, working in pairs, fill in the story map that I have given to you.