Nov 28 should have been Dec 28.
If you have any questions on sex education, then just ask (privately if necessary)
"(making 28 Nov a possible fertile day)" should have been: (making 28 Dec a possible fertile day)
Yes, if she gets a normal period this January, it would mean she is not pregnant.
What if she missed her period? What should we do? We can't go to the doctor because our parents will kill us if they discovered we did that.
How will you know if she has missed her period? You don't know her period history. However, if her last cycle had been 44 days, then if her period has not arrived by 20 Jan (44 days after 7 Dec and also 23 days after 28 Dec), you could take a test which by then should be reliable.
Decide between yourselves what your boundaries are, and keep well within them.
Governed by hormones, one of a woman's 2 ovaries alternatively releases a ripe egg each cycle. This is known as ovulation. If this egg meets sperm in her fallopian tubes on Ovulation Day (OD), then one sperm could fertilise the egg.
After about 6 days the egg arrives in her uterus (womb) and about 3 days later (about 9 days after OD) it would try to adhere to the prepared lining of her womb. That adherence is known as Implantation. That is the point where she becomes pregnant. The egg then receives nutrients and oxygen from the mother, enabling it to grow.
If she does not become pregnant, then the womb-lining (prepared before OD) sheds its preparation about 14 days after OD, and this is the 'menstrual flow' that is discharged through her vagina. That is her period. Her womb then starts its preparation for pregnancy again.
The time from the start of her period up to OD varies considerably but is often 14 days. That 14 days plus the 14 days from OD to her next period would give the typical 28-day cycle (hence the terms 'monthly' and 'menstrual' with links to lunar months).
As the start of the menstrual flow is so obvious, the menstrual cycle is generally reckoned from that day. That start-date is, therefore, Cycle Day 1 (CD1). This means that with a 28-day cycle, OD would be CD15, and her period would start CD 29 (the new cycle's CD1).
Sperm can live in a woman's body for up to about 6 days, so she is considered fertile on OD plus the previous 5 days. With a standard 28-day cycle, she would be considered fertile CD10-CD15.
If the first phase of her cycle is 2 days longer, then OD would be CD17, and her period would be expected to start 14 days later on CD31 (a 30-day cycle). She would then be considered fertile CD12-CD17. Some such women say "I am always 2 days late" because they relate to the typical 28-day cycle, but their cycles are always 30 days long. They are not late, but on-time for a 30-day cycle.
Many women do not have regular cycles, so cannot predict their fertile days with any certainty..
This is a brief and basic description of the menstrual cycle. I hope it helps as a start.