Should schools offer junk food?

As regards to whether schools should offer junk food, there has been a heated public discussion among the public in our society.
Just as every coin has two sides, there is no exception for junk food. On the one hand, it is convenient to make. Whenever you need to eat, junk food is quick and easy to get. ,they can offer you faster than others. One more thing In addition, mostly junk food is more popular than healthy food because some students they don't like to eat vegetables. On the other hand, junk food bring is a bad influences for on our body, especially for students who is are growing up.And If students can have a balanced diet, then they will have a healthy body, which plays an important role in life. With the remarkable advancement of society, more and more people start begin to focus on a healthy body. When it comes to school, every student should have a develop good habits in everything, including eating.
As far as I am concerned, under no circumstances should schools undervalue underestimate the power of building a healthy eating habits. All in all, the schools shouldn't offer junk food.

Nice job! There were a few issues with correct pronouns, plurals, and correct space placement for periods and commas. Other than that, most corrections were simply for flow or vocabulary.

Script of my speech (1)

Good evening (everyone). I'd like to start by introducing myself(. My) name is Carmen Marín. The subject of my presentation is the Mar Menor, (its) importance(,) and (its) recent environmental problems. Have you ever been there? When I was a child, parents always took their sons and (daughters) to this (beach), because of (its) warm and calm water(. It was a perfect place: safe and enjoyable.) Why am I speaking in (the) past (tense)? (That is because) the situation has changed radically. I have chosen to speak about this (topic) because I (am) from San Javier, next to the sea(. This) year’s media is (focused on) the contamination of the Mar Menor and (it) is a topic of habitual conversation in the (region).

I have divided my presentation into 4 parts. In the first part(,) I (will provide) brief information about (this) place(. In) the next section(,) I will explain the importance of the Mar Menor as a tourist point of the region and (its) biodiversity. In part three, I am going to show the causes and consequences that (have provoked) (its) current state. Finally, I (plan) to comment (on) the possible solutions that (have been) proposed to solve this environmental catastrophe. Now let us turn to point one.

(First) of all, I (am sure) that all of you (know) where Mar Menor (is) situated, but let me do a little presentation (either way). Mar Menor is in the southeast (part) of the region, and is surrounded by San Javier, San Pedro del Pinatar, Cartagena, (and) Los Alcázares. About (its) dimensions, (it is important to know) that it has (a surface area of) 135 km2, 73 km of coast(,) and an average depth of 4m. Its average temperature is 15º, being able to reach 30º in the hottest days of the summer. Now, I (am) going to talk about (Mar Menor's) economic and ecological importance.

Nice job on the first part of your speech! You mostly just had errors with run-on sentences, pronouns, and verbs. I was being picky with a lot of the other things.