How to select item in collectionview programmatically?

From your question I'm assuming you only ever have one selected student, I did a similar thing with a collection of icons the user could select. Firstly in view did load I did:

override func viewDidLoad() {
    super.viewDidLoad()

    iconCollectionView.delegate = self
    iconCollectionView.dataSource = self
    iconCollectionView.allowsMultipleSelection = false
    iconCollectionView.selectItemAtIndexPath(NSIndexPath(forItem: 0, inSection: 0), animated: false, scrollPosition: .None)
}

Here I default to selecting the first cell, you would use StudentArray.indexOf to get your selected student index. Then to show the which item is selected I did:

func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
    let cell = iconCollectionView.dequeueReusableCellWithReuseIdentifier(reuseIdentifier, forIndexPath: indexPath) as! IconCollectionViewCell
    cell.imageView.image = UIImage(named: imageResourceNames.pngImageNames[indexPath.row])
    if cell.selected {
        cell.backgroundColor = UIColor.grayColor()
    }
    return cell
}

This is called when the collection is first displayed, then to react to changes in selection:

func collectionView(collectionView: UICollectionView, didSelectItemAtIndexPath indexPath: NSIndexPath) {
    collectionView.cellForItemAtIndexPath(indexPath)?.backgroundColor = UIColor.grayColor()
}

func collectionView(collectionView: UICollectionView, didDeselectItemAtIndexPath indexPath: NSIndexPath) {
    collectionView.cellForItemAtIndexPath(indexPath)?.backgroundColor = UIColor.clearColor()
}

Obviously there are many ways to show cell selection, my way is simple but that's all I needed to do.

EDIT: Since posting the above I have realised it's simpler to just add an observer to selected in the cell class:

class IconCollectionViewCell: UICollectionViewCell {

...

    override var selected: Bool {
        didSet {
            backgroundColor = selected ? UIColor.grayColor() : UIColor.clearColor()
        }
    }
}

With this in place the there is no need to handle didSelect or didDeselect or check for selected in cellForItemAtIndexPath, the cell does it automatically.

You can simply use this after [self.collectionView reloadData]

[self.collectionView 
   selectItemAtIndexPath:[NSIndexPath indexPathForItem:index inSection:0] 
                animated:YES
          scrollPosition:UICollectionViewScrollPositionCenteredVertically];

where index is the index number for the selected student.

Swift

let indexPath = self.collectionView.indexPathsForSelectedItems?.last ?? IndexPath(item: 0, section: 0)
self.collectionView.selectItem(at: indexPath, animated: false, scrollPosition: UICollectionView.ScrollPosition.centeredHorizontally)

Super late I know, but just for Record, for item x in section y:

 NSIndexPath *indexPath = [NSIndexPath indexPathForItem:x inSection:y];

if you want to just highlight an item programmatically, you need to call selectItemAtIndexPath method on the collection view itself like:

[theCollectionView selectItemAtIndexPath:indexPath
    animated:YES scrollPosition:UICollectionViewScrollPositionCenteredVertically];

if you want to simulate selecting an item programmatically and you get the code in did select gets called, you need to call selectItemAtIndexPath method on the view controller itself and pass to it your collection view as param like:

[theViewController collectionView:theCollectionView didSelectItemAtIndexPath:indexPath];

Now as sure thing, in order to get an item selected and the did select code gets called, you need to call both :D

[theCollectionView selectItemAtIndexPath:indexPath
    animated:YES scrollPosition:UICollectionViewScrollPositionCenteredVertically];

[theViewController collectionView:theCollectionView didSelectItemAtIndexPath:indexPath];

SWIFT version:

let indexPath = IndexPath(item: x, section: y)
theCollectionView.selectItem(at: indexPath, animated: true, scrollPosition: .centeredVertically)
theViewController.collectionView(theCollectionView, didSelectItemAt: indexPath)

enjoy!