The trouble with these is that the sample size isn't large enough to give a definite answer; there are a number of patterns that might fit the bill. For example:

9    11    _    _    14    8
8 is one less than 9. 11 is 2+9 and 3 less than 14. If we continue this alternating pattern inward, then the middle numbers would be 13 and 8.

6    4    2    5    _    _
This one counts down by twos from 6 until it hits zero, then bounces back to 6-1=5. So, the next two numbers could be 3, 1.

3    7    4    1    8    _
3+4 = 7, so 1+7=8. The missing number may be 7.

18    _    8    _    48    15
These really do look chosen at random. I notice that all the odd numbered places have a last digit of 8, and you're given one even place that has a last digit of 15. So, maybe the even spots should be something like 5 and 35.

I don't know, sam. My patterns fit, sort of. They just didn't give you enough numbers to know for sure. Let's hope your teacher is of the reasonable type that listens to reason.