As a teacher, you have a duty of care to the pupils in your class. Their parents are putting their trust in you and it is important that you have their safety as a number one priority. When you are a teacher, especially if it is a large class, it is easy to become distracted by one pupil if they are in need of help or causing a distraction. When this happens you should be sure that you have a TA or another teacher on hand to help you should you need to leave the classroom or not be able to talk to other pupils. Schools are often understaffed due to budget restrictions, so you may not always have a TA with you. Many schools have a walkie-talkie system or phone system in place to allow you to contact other members of staff quickly should you need to.
When working with items that could be dangerous such as scissors for younger children or chemicals, you may decide to do this in smaller groups rather than a whole class activity. Set other work for the pupils to be getting on with and rotate the groups around so that everyone has a go. This will allow you to keep more control of the situation and access what is going on.