Just take as many shots as you can!... before, during & after...
(this was taken on the football field, (liked her pro studio cap & gown senior pic better but it didn't have the obvious joy of this one) & I was disappointed with the actual graduation shots as I couldn't get close enough & my zoom is lacking... but was satisfied overall)
If you're taking a pre grad shot then you have more control over the setting & lighting which is an added bonus