Nous sommes censés quitter le hall avant midi, mais la réceptionniste nous laisse encore quelques minutes.

Because censé is an adjective here, and it agrees with nous.

• nous = we
• sommes = are
• censés = supposed

Since nous refers to more than one person, censé takes the plural ending -s: censés.

If the group were entirely female, you would usually write censées instead.

So:

• Nous sommes censés... = We are supposed to...
• Nous sommes censées... = We are supposed to... (if all the people are female)

Être censé + infinitive means to be supposed to, to be expected to, or sometimes to be meant to.

In this sentence:

• Nous sommes censés quitter le hall avant midi
  = We are supposed to leave the lobby before noon.

It is a very common French structure.

Examples:

• Tu es censé travailler. = You are supposed to be working.
• Ils sont censés arriver bientôt. = They are supposed to arrive soon.

Because the pattern is:

être censé + infinitive

So after censé / censés / censée / censées, you normally use a verb in the infinitive.

Here:

• être censés quitter = to be supposed to leave

Other examples:

• Je suis censé comprendre. = I am supposed to understand.
• Elle est censée venir. = She is supposed to come.

Both can work, but they are not exactly the same.

• quitter le hall = to leave the lobby/hall
• sortir du hall = to go out of the lobby/hall

Quitter focuses on leaving a place. It takes a direct object:

• quitter le hall
• quitter la chambre
• quitter l’hôtel

By contrast, sortir often needs de / du / de la when you say where you are going out of:

• sortir du hall
• sortir de la chambre

So quitter le hall is a very natural choice here.

Yes. Hall is used in French, especially for a hotel lobby, entrance hall, or large interior space.

It is a borrowed word, but it behaves like a normal French masculine noun:

• le hall
• dans le hall
• quitter le hall

Depending on context, learners may also see words like:

• le lobby = the lobby
• le hall d’entrée = the entrance hall
• la réception = the reception area/front desk

Because in French, times of day used this way usually do not take an article.

So you say:

• avant midi = before noon
• après midi is different, because l’après-midi is a noun meaning afternoon

Here, midi means noon as a point in time, so no article is needed.

Compare:

• avant midi = before noon
• à midi = at noon
• vers midi = around noon

Mais means but.

It introduces a contrast between two ideas:

• Nous sommes censés quitter le hall avant midi
  = We are supposed to leave the lobby before noon
• mais la réceptionniste nous laisse encore quelques minutes
  = but the receptionist is still giving us a few more minutes

So the idea is: the rule says one thing, but in practice the receptionist is allowing a short delay.

Yes. Réceptionniste can refer to either a man or a woman. The noun itself often has the same form for both genders, and the article shows the gender:

• le réceptionniste = the male receptionist
• la réceptionniste = the female receptionist

In your sentence, la tells you the receptionist is female.

Also note the accent in réceptionniste. French spelling keeps that accent because it comes from réception.

Because French object pronouns usually go before the conjugated verb.

Here nous means us, not we.

So:

• la réceptionniste nous laisse = the receptionist leaves us / lets us

The order is:

• subject: la réceptionniste
• object pronoun: nous
• verb: laisse

This is the normal pattern in French:

• Il me voit. = He sees me.
• Elle nous attend. = She is waiting for us.
• Le professeur vous aide. = The teacher helps you.

Not here. Laisser has several meanings, and in this sentence it means to let or to allow.

So:

• la réceptionniste nous laisse encore quelques minutes = the receptionist is letting us have a few more minutes = the receptionist is allowing us to stay a few minutes longer

This is different from laisser meaning to leave something behind:

• J’ai laissé mes clés sur la table. = I left my keys on the table.

French uses the same verb for both ideas, so context matters.

It means a few more minutes or a few minutes longer.

Breakdown:

• encore = still, again, more
• quelques minutes = a few minutes

Together here, encore quelques minutes means the extra time is continuing beyond what was expected.

Examples:

• Attends encore une minute. = Wait one more minute.
• Nous avons encore du temps. = We still have time.
• Il reste encore quelques places. = There are still a few seats left.

So in your sentence, encore adds the idea of extra allowance.

Because French often uses the present tense to describe what is happening now or what is currently the case.

So:

• la réceptionniste nous laisse encore quelques minutes = the receptionist is letting us stay a few more minutes

In English, you might naturally say is letting us, but French often just uses the simple present:

• elle laisse = she lets / she is letting

This is very normal in French.

Not in this exact structure.

With laisser, French often uses:

laisser + direct object + infinitive

For example:

• La réceptionniste nous laisse rester quelques minutes. = The receptionist lets us stay a few minutes.

But your sentence is slightly different:

• La réceptionniste nous laisse encore quelques minutes.

Here quelques minutes is the thing being granted or allowed, so there is no infinitive after laisser.

So both are possible, but they mean things in slightly different ways:

• Elle nous laisse encore quelques minutes. = She gives/allows us a few more minutes.
• Elle nous laisse rester encore quelques minutes. = She lets us stay a few more minutes.

It sounds mostly neutral everyday French.

• Nous sommes censés... is standard and common.
• quitter le hall avant midi sounds natural in a hotel context.
• la réceptionniste nous laisse encore quelques minutes is also standard and natural.

It is not especially slangy, and it is not stiffly formal either. It would fit well in normal spoken or written French.