Schedule the meeting in/on the remaining slots

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As @Edward points out, here are some scenarios to consider. Others may pitch in if needs be.

If you are the person scheduling the meeting, or if you know someone is doing it, you would say, The meeting/a meeting will be scheduled on next Friday at ...

If you are informing/confirming to someone about an already scheduled meeting, you would say, A meeting/the meeting is already on schedule or is scheduled for next Friday.

You may also say, The n slots are available for scheduling a meeting/the meeting on Friday.

To the first blank, it would be appropriate to say, Schedule the meeting in one of the remaining slots.

You see, rather than the preposition, it is also about the expression in the context. Don't overthink. Noone can fine you for being on schedule!!!