[Last updated: 18th February 2020]
I’ve tried to find all of the Schools which include masters students and below, not only PhD students. However, if you happen to be a PhD student, look here and here for tons more opportunities.
Europe
OPTICON – [New!] Observing school at the Observatoire Haute Provence, October 2020.
Middle East / North Africa
AWSA – The Arab Winter Schools for Astrophysics take place in the Middle East / North Africa in October each year.
Americas
La Serena – A week long school in August, in Chile, which focuses on Applied Tools for Astronomy.
ISYA – The International School for Young Astronomers 2020 will take place in Nov/Dec in South Africa for three weeks. Anyone from senior undergraduates to postdocs can apply.