Creating Unique Alpha Numeric Codes

I am very new to programming and have recently been learning Javascript.

I have a project that I would like to complete that involves the following:-

1) Generating a set number of unique Alpha numeric codes (these will be done in various batch sizes from 10 - 3000), they must also be unique.

2) convert the Code to a QR Code to be appended to a PDF (for later photographing)

3) A school photo shoot will be carried out using the QR Code where each child is photographed with one code.

4) Images are then processed by the webiste and will take the QR Code Alpha numeric and tag it to the image.

5) The images will then be uploaded to a separate website and the original Alpha numeric will be used by the parent to view just their childs images.

So the Question........

What is the best way to ensure that the Alpha Numeric remains unique across all possible combinations?

1) Should I create a text file on the server that holds each previously created code to be checked each time a new one is created.

2) Put the previously assigned codes in a back end database to be checked each time.

3) Some other method that I need to research...?

It is important that each child's image has a unique password otherwise parents would potentially view children that are not their own if a code is ever duplicated.

And for information the reason that I am using Javascript is that eventually I would like to build in the functionality of having my own developed Online viewing website for the images.

Thank you in advance for helping.

Regards