Во время практики в школе я буду объяснять грамматику детям.

Во время means during.

The basic preposition is в, but Russian sometimes uses во instead for easier pronunciation. Here, в время would be awkward to say, so it becomes во время.

Also, во время is a fixed expression, and it requires the genitive case after it:

• во время урока = during the lesson
• во время каникул = during the holidays
• во время практики = during the internship / practical training

Because во время requires the genitive case.

The dictionary form is практика, but after во время it changes:

• практика → nominative
• практики → genitive singular

So:

• Во время практики = during the internship / during the teaching practice

Here практика means something like practical training, placement, internship, or teaching practice, depending on context.

Since the sentence also says в школе, it most likely refers to a student teacher’s school placement or teaching practicum.

So this is not practice in the English sense of repeating a skill. It is more like a period of supervised practical work.

В школе means in school or at school, depending on how naturally you translate it into English.

After в meaning location, Russian uses the prepositional case:

• школа → dictionary form
• в школе → in/at school

Here it tells us where the практика takes place:

• Во время практики в школе = during my practical training at school

Because объяснять is an imperfective verb, and imperfective verbs form the future with быть + infinitive:

• я буду объяснять = I will be explaining / I will explain
• ты будешь объяснять
• он/она будет объяснять, etc.

This kind of future is used when the action is seen as:

• ongoing,
• repeated,
• part of a process,
• or not focused on completion.

In this sentence, the idea is that during the teaching practice, the speaker will be engaged in the activity of explaining grammar to children, possibly more than once.

This is a very common Russian aspect question.

• буду объяснять = future of the imperfective verb объяснять
• объясню = future of the perfective verb объяснить

The difference is aspect:

• буду объяснять focuses on the process, repeated action, or general activity
• объясню focuses on completing the explanation

So:

• Во время практики в школе я буду объяснять грамматику детям
  = During my school placement, I’ll be explaining grammar to children.
  This sounds natural because it describes a teaching activity over a period of time.

If you said:

• я объясню грамматику детям it would sound more like:
• I will explain the grammar to the children
  with emphasis on getting that explanation done.

Because грамматику is the direct object of объяснять, so it takes the accusative case.

• грамматика = nominative, dictionary form
• грамматику = accusative singular

You explain something, so that thing is in the accusative:

• объяснять грамматику
• объяснять правило
• объяснять тему

Because детям is the dative plural of дети and marks the person receiving the explanation.

With объяснять / объяснить, Russian often uses this pattern:

• explain something = accusative
• to someone = dative

So:

• объяснять грамматику детям = to explain grammar to children

Compare:

• Я читаю книгу детям = I am reading a book to the children
• Она показывает картинку студентам = She is showing a picture to the students

The singular is:

• ребёнок = child

But the common plural is irregular:

• дети = children

Its dative plural form is:

• детям = to children

This is one of the important irregular noun patterns to learn:

• дети → детям
• люди → людям

Yes. Russian word order is fairly flexible because case endings show the grammatical roles.

The sentence:

• Во время практики в школе я буду объяснять грамматику детям

is a neutral, natural way to say it.

But other orders are possible, depending on emphasis:

• Я буду объяснять детям грамматику во время практики в школе.
• Грамматику я буду объяснять детям во время практики в школе.
• Детям я буду объяснять грамматику во время практики в школе.

These all mean roughly the same thing, but the emphasis shifts:

• first position often gets special attention,
• the end of the sentence often carries new or important information.

Yes, often it can.

Russian frequently leaves out subject pronouns when the verb ending already makes the subject clear. Since буду clearly means I will, you could say:

• Во время практики в школе буду объяснять грамматику детям.

That is grammatical and natural in the right context.

However, я is often kept when:

• you want clarity,
• you want contrast,
• or you want slight emphasis on I.

Both are possible.

Russian allows both:

• объяснять грамматику детям
• объяснять детям грамматику

Both mean to explain grammar to children.

The difference is mostly about emphasis and information flow, not basic grammar. In many contexts, объяснять детям грамматику may sound slightly more conversational, but объяснять грамматику детям is completely normal.

Most naturally, it describes практики:

• Во время практики в школе
  = during my practical training at school

So the whole opening phrase sets the time-and-context frame for the sentence.

Of course, being at school is also where the explaining happens, so semantically it connects to the whole situation. But structurally, a learner will usually understand it as the practical training that takes place in a school.

A useful breakdown is:

• Во время практики в школе = during my school placement
• я буду объяснять = I will be explaining
• грамматику = grammar
• детям = to children

So the pattern is:

time expression + subject + future imperfective verb + direct object (accusative) + indirect object (dative)

More abstractly:

• During X, I will explain Y to Z

This is a very common Russian structure, and it is good practice for:

• во время
  • genitive
• future of imperfective
• accusative for the thing
• dative for the recipient