Zas was selected by the to represent them at the Under-18 Craven Week competition held in Polokwane in 2013. He started three matches for them at outside centre and scored 18 points for his side, which consisted of one try in their match against hosts and 13 points with the boot. His performances were also noticed by the selectors of the South African Schools team and he made two appearances for them in the 2013 Under-18 International Series; he started both their 17–13 victory against France and match against Wales, as a try by Zas proved crucial in a 14–13 victory.
After high school, Zas joined the Western Province Rugby Institute and he represented in the 2014 Under-19 Provincial Championship. Zas started eight of their twelve matches during the regular season and scored five tries – three of those came in a single match against, while he also scored against in both their home and away matches in the competition. Western Province finished in third spot on the log to qualify for the semi-finals, where they met the Free State U19s and Zas scored for a third time against the same opposition in a 29–22 victory in Bloemfontein. He also started the final, where Western Province became champions by beating the s 33–26. In 2015, he was selected a 37-man South Africa Under-20 training squad and started for them in a friendly match against a 2015 Varsity CupDream Team, scoring a try in a 31–24 victory. He was named in their squad to tour Argentina for a two-match series as preparation for the 2015 World Rugby Under 20 Championship. He started both their 25–22 victory over Argentina in the first match, and their 39–28 win in the second match four days later, scoring a second-half try for the visitors. Upon the team's return, he was named in the final squad for the 2015 World Rugby Under 20 Championship. He started all three of their matches in Pool B of the competition; a 33–5 win against hosts Italy, a 40–8 win against Samoa which saw Zas score a try shortly after half-time and their final match against Australia, with Zas again on the scoresheet in a 46–13 win. The results meant that South Africa finished top of Pool B to qualify for the semi-finals with the bestrecord in the pool stage of all the teams in the competition. Zas also started their semi-final match against England, but couldn't prevent them losing the match 20–28 to be eliminated from the competition by England for the second year in succession. He started their third-place play-off match against France, helping South Africa to a 31–18 win to win the bronze medal.
In 2016, Zas was included in a South Africa 'A' squad that played a two-match series against a touring England Saxons team. He was named in the starting line-up for their first match in Bloemfontein, but ended on the losing side as the visitors ran out 32–24 winners. He also started the second match of the series, kicking one conversion in a 26–29 defeat to the Saxons in George.
