How can a teacher keep the classroom engaged?

A teacher must be a student centered in other word you have to make sure that everyone in the classroom should participate and interact.

Make the subject interesting, and present it in many different ways, so you don't make the students bored of the topic, and let a space for their participation; don't be just an instructor; give them somthing to work on in the class.
And be sure that every student has his share from the session time, let everyone involved according to his abilities.

A teacher is an instructor and a facilitator, so give a good presentation with the related subject matter and provide the similar task as class work where students can get enjoyment to give answers.
If you do so, then you can get your class engaged with class work.

Oh! only two techniques.
One be attentive and another be responsive.
They will be engaged in classwork activities.

Most important thing is to have thorough knowledge of the topic you are teaching so that you are never short of confidence on that count...besides that explaining expertly what you know is also very important...communication skills come into play here..pitch, voice modulation are important...and besides this engaging students by making them participate in discussion is also critical to maintain their interest.

student centered approach is necessary in a classroom teaching to keep our students engaged in our lesson however, we have to be consider about their learning capability differences so i would say that DCIA or Differentiated Curriculum Instruction and Assessment strategy is also need to be consider to keep the classroom engaged.

Group discussions, general topics, mind games.
So if the teacher can incoporate this into the hour or so, attention span is retained to the max, as it is more like refreshing their concious mind.

Attract the attention of the student through explanation - the debate - the diversity of styles annotation using available hardware to make the students involved in the process of explanation -

By using teaching strategies i.e.
Group Discussion, pair work, Whole class discussion etc.

A teacher can keep the classroom engaged in many ways.
First, before starting the activity/lesson, he should prepare the students for it so that they get eager to go through the activity/lesson.
Secondly, teacher should make the lesson as interesting as possible, by using audio-visual aids in class.
Students should then be assigned tasks according to the requirement of activity, and teacher,as a motivational force, should facilitate the students in completion of the tasks.
Every student should be given his/her separate task, even working in a group, so that he should not only be engaged but is also aware that he/she is doing something to prove his/her worth.

student should look for thier lessons and work hard because they are interested this can be achieved through different motivation techniques at the same time the teacher has to look for the blocks a student may have against learning .
Additionally,a class may need some hing personalization and most student will envolve themselves when a lesson allows them to talk about their interests and the nearer language teaching can come to real life the more interesting it will be and allowing students to perform short sketches inside class will increase their confidence about doing that in real life .
In short there is probably no one perfect way of keeping ur classroom engaged but rather different ways and techniques.