First of all - welcome to BC !
It is not that bad a photo, yes there is a beam of sunlight coming diagonally left to right across the picture which has slightly raised the exposure on those people caught by it. The forehead of the older man in the middle and the cheeks of a boy in the front row are probably the worst.
I do not know Pixelmator so I have no ideas what tools are available but this is a very quick and dirty job I did on the forehead of the older man. Try blowing up the image as large as you can, certainly until you can see the pixcels in it, then use the colour picker tool to select a suitable skin tone then paint it into the over-bright bits changing the colour as needed. Since these tools are available in the Windows Paint application they should be available in your application.
NOTE - Make a copy of the image and work on the copy. this way, if it goes wrong you still have original to make another copy from and try again !