I've been told by a high school choral director that drinking sweet drinks, such as soda, before singing is not not healthy/good for the voice. Is this true? If so, why is it bad for you?
It is often not sweet drinks, per se, but what's in the most popular sweet drinks.
So for example, soda (I assume the UK equivalent would be a 'soft drink' like Coca Cola?) often contains caffeine, which has a dehydrating effect and therefore affects the voice. This was borne out by a study in 1999 'Effect of caffeine on the vocal folds: a pilot study'* which (if I understand the abstract correctly) concluded that caffeine does produce an alteration in voice quality but the effect can vary for each person.
The British Voice Association recommends steering clear of alcohol, coffee, tea and cola. Their voice care leaflet is at: http://www.british-voice-association.com/downloadable-resources.htm
Any drink that is a diuretic (regardless of caffeine) can cause problems because if you are urinating more often, you're losing valuable water - and staying hydrated is important for a singer.
Sweet drinks that contain milk can also cause problems if drunk just before singing because dairy products create thick mucus, making it harder to sing.
So, in summary, I haven't seen anything that specifically points to a problem with a drink being 'sweet' (or containing sugar) but it's perhaps more likely that those drinks have other ingredients that may affect a singer. As a final thought, singers ideally should maintain a good level of fitness, so cutting down on drinks with lots of sugar may be part of that maintenance!
*Ref: Akhtar, Wood and Rubin at the Department of Speech and Language Therapy, Royal National Throat, Nose and Ear Hospital, London, UK.
There are two potential problems for "sweet drinks" (more likely, carbonated drinks)
Milk is a bad choice because it causes mucus buildup.
In summation, water is the best choice for optimal vocal performance. You can get away with drinking carbonated drinks or caffeinated drinks, but you may find notes hard to sing as you feel a burp coming up. Not to mention, sweet drinks are loaded with sugar, so you don't need them anyway!