Reír con mis amigos en la escuela me hace feliz.

Because reír is being used as an infinitive, and in Spanish an infinitive can act like a noun or a whole activity.

So Reír con mis amigos en la escuela means something like Laughing with my friends at school.

That whole infinitive phrase is the subject of the sentence, and then me hace feliz means makes me happy.

A similar pattern in English is:

• Reading helps me relax.
• Swimming is fun.

Spanish does this very naturally with infinitives.

The accent mark shows where the stress goes and helps mark the vowel pattern.

Reír is pronounced with stress on the -ír part: re-ÍR.

Without getting too technical, the accent helps show that the i and e are pronounced separately enough for the word to need written stress marking. This is just part of the standard spelling of the verb:

• reír = to laugh
• sonreír = to smile

So this is simply the correct dictionary form of the verb.

Both are possible, but they do not sound exactly the same.

• mis amigos = my friends
• amigos míos = friends of mine / my friends, with more emphasis

In neutral, everyday Spanish, mis amigos is the normal choice before the noun.

So:

• con mis amigos = the most natural basic phrasing

If you said con amigos míos, it would sound more like with some friends of mine.

In Spanish, you normally do not use an article before a possessive adjective like mi, mis, tu, su, nuestro, etc.

So Spanish says:

• mis amigos

not

• los mis amigos

This is different from English only in form, not in meaning.

Literally, it means it makes me happy.

Here is the structure:

• me = me
• hace = makes
• feliz = happy

The subject of hace is the whole phrase Reír con mis amigos en la escuela.

So the logic is:

• Laughing with my friends at school = subject
• makes me happy = verb phrase

That is why hace is in the third-person singular form: the whole activity is treated as one thing.

Because the subject is not mis amigos. The subject is the whole infinitive phrase:

• Reír con mis amigos en la escuela

That whole idea counts as a single activity, so Spanish uses singular hace.

Compare:

• Reír con mis amigos me hace feliz.
  = Laughing with my friends makes me happy.

But:

• Mis amigos me hacen feliz.
  = My friends make me happy.

In the second sentence, mis amigos really is the subject, so the verb is plural: hacen.

Because en means in / at, while a usually shows movement to a place.

Here, the sentence is talking about where the laughing happens:

• en la escuela = at school / in school

If you said a la escuela, that would suggest motion toward the school:

• Voy a la escuela. = I go to school.

So in this sentence, en is the correct preposition.

It is understandable, but in Spain the most natural word often depends on the type of school.

Common choices in Spain are:

• colegio = primary school / general school, often very common for younger students
• instituto = secondary school / high school
• escuela = school, but often used in certain contexts like escuela de idiomas, escuela de música, etc.

So although en la escuela is correct Spanish, a speaker from Spain might often say something more specific, such as:

• en el colegio
• en el instituto

depending on the situation.

Yes, you may hear both reír and reírse, but the meaning can shift depending on context.

• reír = to laugh
• reírse = to laugh / to have a laugh, very common in everyday use

For a general idea like this sentence, reír works well.

Be careful, though, because reírse de alguien means to laugh at someone:

• Se ríe de mí. = He/She laughs at me.

So in your sentence:

• Reír con mis amigos... = to laugh with my friends

is clear and natural.

Because con is the normal, natural preposition for with when doing something together.

• reír con mis amigos = laugh with my friends

junto a usually means next to / beside, and it often emphasizes physical position more than companionship.

So:

• con mis amigos = doing the action together
• junto a mis amigos = physically next to my friends

In this sentence, con is the better choice.

Yes. Spanish word order is more flexible than English, although some versions sound more natural than others.

Original:

• Reír con mis amigos en la escuela me hace feliz.

You could also say:

• Me hace feliz reír con mis amigos en la escuela.

This version is also very natural and maybe even a bit more common in everyday speech.

Both mean the same thing. The difference is mostly one of style and focus.

In this sentence, the most natural interpretation is that the laughing happens at school.

So the phrase en la escuela modifies the activity:

• reír con mis amigos en la escuela

In real life, context decides exactly what is meant, but a listener will normally understand it as:

• I laugh with my friends while I am at school.

If you needed to be extra clear, you could rephrase it, but the original is already understandable.

Not if you want to say makes me happy.

• me hace feliz = makes me happy
• hace feliz = makes happy

Without me, the sentence feels incomplete unless you add some other object:

• Reír con mis amigos hace feliz a mucha gente.
  = Laughing with friends makes many people happy.

So in your sentence, me is necessary.