Figure out techniques that work best for your type of stuttering. I tend to get blocks rather than repetitions, so focusing on my breathing and light articulatory contact helps me best (not clenching my jaw, or pressing my lips together too hard, just barely touching my tongue to the back of my teeth or wherever it needs to go to make the sound). And most importantly, never give up. You might lose the battle, but you must win the war against your stutter! Don't beat yourself up or sell yourself short. I'm not sure about how the speech test is designed or disability rights your country, state, school has, but maybe you could tell the principal or the teacher what happened and they could let you retry the speech in a setting that is more comfortable for you... Who knows.