1) My family and I have been on holiday in Greece. We stayed on a little island called ... There wasn't any wind. Unfortunately, we spent only a week there.
2) I have just finished my dinner. I finished having dinner. <~~Both are OK.
3) My family and I went to Rome last month.
4) School starts on Monday. I'm sorry we haven't been in touch recently.
She is 15 years old, just like us. <~~OK
I was very sad because I still had to do my homework and study. <~~OK
What was your holiday like? How was your holiday? <~~Both OK
5) I hope to receive an answer soon (your prompt reply, a quick reply from you'') Keep me up to date with what happens to you. <~~All are OK.
6) I still have a lot of things to study because school starts tomorrow. <~~OK